Cache results of reference tests

[lemmy]

To reduce the runtime of the test suite, we could cache the results of the reference tests, such as Apalache and ref TLC. This can be achieved using the GitHub Actions cache feature: https://github.com/actions/cache

[lemmy]

@craft095 Do you think it would work if we removed the TLC part of the test results before caching, and only kept the other results?