Change tests to use save files not in the main repo

[chennin]

Change how test saves are stored to address main repo size concerns

• Change the saves directory to a submodule repo I created (I will add collaborators, or, I don't need to be the one to own it)
• Change the real_save test to use a temp directory that pytest will automatically clean up
• And fail if zero output .dbs are created (it'd take more work to automate that number, complicated by the fact that if we ever put multiple saves from the same game in, they will go to the same db)
• Check the submodule out in the github action
• List tests more verbosely for when we want to manually confirm one ran
• Tests pass and output artifacts are the same size (example, example)

[chennin]

@eliasdoehne here's a solution to adding more test saves without bloating the size of the main repo - what do you think?

[MichaelMakesGames]

Looks good to me. I'll wait a bit before merging to give Elias a chance to respond

[eliasdoehne]

Looks good to me 👍