I'd like to test my in-development Card Game, can I do this with Roll 20?

It depends on what sort of gaming mechanics you need to do with a deck(s). Things you can currently do: Players can hold cards from multiple decks in their hand. Players can trade cards. You can have and use multiple decks in a campaign. You can set up whether the card fronts are visible to everyone or to no one or only the person who is holding the card. There is a discard pile. You can set a default on how a card is facing when you pull it to the tabletop. Things you can't, as yet, do: Look through the deck for a specific card Look through the discard pile for a specific card Place a card back in the deck in a specific place (bottom of the deck, after a certain number of cards, on the top, etc.) Deck building outside of having all the desired cards kept in a player's hand. Have a card deck that has different fronts AND backs

Welcome! I'm sorry that the card deck feature still lacks what you need to run your testing. The Developers readdress features in passes, so hopefully later on there'll be a more robust set of card deck features in the future to utilize.