Unify test resources

[loosebazooka]

This is now in progress

• [ ] share common test classes (#2350)
• [ ] share common test resources
• [ ] ... and

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We should share as many test resources as possible. This means

1. putting all test resources in a common location
2. rewriting our TestProject fixtures to access them
3. fix tests to address any deviation in test projects that have happened over time.
4. and so much more!!!

[TadCordle]

Would be nice to streamline the plugin integration test code a bit more as well in general. The tests themselves work fine, but they operate using a lot of awkward halfway-generic helper methods and classes. Some of the methods cover multiple cases, but often require further modification/rewrites when we introduce new tests (e.g. I feel like every time I have to add a new integration test, I have to reread a lot of code to re-learn how it works and end up adding new flags to some helper methods or something to fit the new use case).

Almost feels like we should either just be copy-pasting the code in the tests so they're easier to read/follow, or make a more general/flexible/future-proof testing framework.