Add additional testing

[nickelization]

I'd like us to write more tests for the framework itself, and verify that stuff like shrinking works correctly. Not super critical, since the test suites that use proptest-stateful kiiiiind of indirectly act like tests for the framework itself...but would still be nice to have unit tests/proptests/whatever so that we can be more confident of not missing any bugs, and so that we can have higher confidence of avoiding regressions when we make changes to this crate.