English Test Print Pokémon Cards
This is a list of well-recognized English Pokémon test print cards. These are not all the same type of testing. Some were used to test a stamp, some appear to be holofoil tests, and some were internal production cards not meant for normal distribution.
There are other types of 1-off test prints where the card and situation is truly unique.
List of cards
Prototype Blastoise
A test print of a Blatoise Pokémon card on Magic: The Gathering card stock. The card was printed as a test by Wizards of the Coast during the early stages of Pokémon card production. It features the artwork of Blastoise from the Japanese promo, but is printed on the thicker cardstock used for Magic: The Gathering cards. The original foil version of the card was authenticated by a lead typesetter at Wizard of the Coast. A galaxy foil version has been authenticated by CGC, but lacks a similar strong provenance from Wizards.

Prerelease Test Print Raichu
The Prerelease Test Print Raichu is the most famous English Pokémon test print card. It is a version of Base Set Raichu believed to have been used to test the "Prerelease" stamp before that stamp was used in production on the Jungle Clefable prerelease promo.
The card is distinctive because it combines a normal Base Set Raichu with a stamp that does not belong on any released version of the card. Sources commonly describe almost all copies as destroyed, leaving only a very small number surviving and likely kept by Wizards of the Coast employees.

Base Set Disco Holo test prints
The Charizard Disco Holo, Blastoise Disco Holo, and the other Base Set holo rares in the "Disco Test Prints" group are assumed early holo test prints.
Square-cut versions were likely given only to Wizards of the Coast employees, and then unintentionally circulated through uncut sheets. There is hear-say that square-cut versions were placed as orniments during a holiday party and several employees took them home. The easiest way to distinguish them is by the disco-style foil pattern instead of the normal released holofoil treatment. There is no Machamp disco holo.

The currently represented cards in the English Pokémon database on TopVault are:
Alakazam, Blastoise, Chansey, Charizard, Clefairy, Gyarados, Hitmonchan, Magneton, Mewtwo, Nidoking, Ninetales, Poliwrath, Raichu, Venusaur, Zapdos
Note that additional "disco holo" cards, such as a 1st Edition Charizard, and uncut sheet with a Machamp exist but have not been authenticated by Wizards, nor any grader or authenticator.
Base Set Cosmos Holo test prints
The Charizard Cosmos Holo, Blastoise Cosmos Holo, and the other Base Set holo rares in the "Base Cosmos Holo" group appear to be a later round of prototyping.
They were likely early prototyping for Base Set 2, and not intended for distribution. Like the Disco Holo cards, these also seem to have circulated from uncut sheets rather than normal pack release. The most obvious difference is the foil treatment, using a cosmos holo pattern instead of the normal Base Set holofoil. The Machamp is not a test print, but rather was widely circulated in the 2-Player CD-ROM game.

The currently represented cards in the English Pokémon database on TopVault are:
Alakazam, Blastoise, Chansey, Charizard, Clefairy, Gyarados, Hitmonchan, Magneton, Mewtwo, Nidoking, Ninetales, Poliwrath, Raichu, Venusaur, Zapdos
Jungle White Border test prints
The Jungle set also has a group of White Border test prints. Unlike the Base Set and Fossil holo test cards, these are non-holo rares and are distinctive because they have a plain white border instead of the normal colored border.
There is not much public documentation about the origin of these cards. The clearest public evidence is from surfaced examples and collector discussion, where square-cut cards with normal backs have appeared. That suggests these came from test sheets and were not normal pack-issued cards, but there does not seem to be a strong public explainer for exactly why they were made.

The currently represented cards in the English Pokémon database on TopVault are:
Clefable, Electrode, Flareon, Jolteon, Kangaskhan, Mr. Mime, Nidoqueen, Pidgeot, Pinsir, Scyther, Snorlax, Vaporeon, Venomoth, Victreebel, Vileplume, Wigglytuff
Fossil Cosmos Holo test prints
The Fossil set also has its own group of Cosmos Holo test prints. These are believed to be test print cards that were not made available for normal distribution. These all have a "1999 Wizards" copyright.

The currently represented cards in the English Pokémon database on TopVault are:
Aerodactyl, Articuno, Ditto, Dragonite, Gengar, Haunter, Hitmonlee, Hypno, Kabutops, Lapras, Magneton, Moltres, Muk, Raichu, Zapdos
Like the Base Set Cosmos Holo cards, these are easy to overlook at a glance because the fronts are still very familiar. But the holo pattern and the copyright detail separate them from standard Fossil releases.
The Zapdos test print is a good example of this. The test print has the Cosmos holo pattern and specifically has a "1999" copyright. This should not be confused with the Fossil Zapdos "1999-2000" version from the theme deck shown below on the left, which is a different card and not a test print on the right.

Expedition For Position Only cards
The Expedition For Position Only Alakazam, Charizard, and the other cards in this group are another kind of internal test card.
In TopVault these are described as cards printed with "For Position Only" text and likely used in filler or gap positions on large print sheets. The notes also describe them as cards inserted into unreleased internal test packs. Those packs reportedly used Legendary Collection artwork because Expedition was not yet released.
These are especially interesting because they are not just alternate foil patterns. They are likely internal layout or production cards that were never intended to survive at all.

The currently represented cards in the English Pokémon database on TopVault are:
Alakazam, Ampharos, Arbok, Blastoise, Charizard, Clefable, Dugtrio
Further reading
If you want more background on these cards, or additional collector discussion and authentication details, these are useful references:
A Pokémon TCG Myth No More: CGC Cards Certifies Two Prerelease Raichu for the history, provenance, and authentication work behind Prerelease Raichu.
CGC Cards Certifies Several Cosmos Holo Test Print Pokémon Cards for a strong explainer on why English Base Set Cosmos holo cards are considered test prints.
Rare English Cosmos Fossil Holofoil Error? Test Run? Misprint? for collector discussion specifically focused on the English Fossil Cosmos holo cards, including the Zapdos copyright distinction.
Probstein123 Auctions 50 White Border Jungle Test Prints for a collector discussion thread showing surfaced examples of the Jungle White Border test prints. This is more of a sightings and market thread than a full explainer, but there is not much public documentation on these cards.
FPO - For Position Only Cards Official Guide for a detailed history and identification guide for the Expedition For Position Only cards.
Masters Guide for Pokémon WOTC Corrected Errors & Test cards for a broader long-term reference covering many WotC-era errors and test cards in one place.
Wrap up
These English Pokémon test print cards are not all one category of card. Some were used to test a stamp, some tested alternate holofoil patterns, some appear to be border or layout tests, and some were internal production cards that were never meant to leave the building.
This is what makes them especially interesting. They are not just rare versions of already-known cards. They show different stages of the card production process, and they preserve details that were removed before final public release.
If you know of additional English-language Pokémon test print cards that should be included here, please reach out at teddy@vault.top, or on the TopVault Discord.