Improve snapshot test suite to cover more edge cases

[PullJosh]

Our tests just caught a bug, which is pretty neat! I'd like to reach a point where automated tests can pretty much be trusted, so that if the tests pass, the PR can be merged. To do so, we need a better corpus of wonky projects.

Fortunately, such a corpus has already been compiled for just such a situation as this!

I think it makes sense to take most of the projects in the corpus and include them as snapshot tests. (Once SB3 output lands, we can even create tests that expect any .sb3 that is inputted and then regurgitated back out to match the original exactly!)

[towerofnix]

This is a pretty big project and relevant to stuff I want to work on (improving testing in general), so I'm self-assigning for this! I've retitled the issue to address snapshot tests in particular, since we may want to implement unit tests later, which are an entirely separate thing to track.