Clean up Tact's testsuite

[anton-trunov]

For instance, the grammar and typechecker tests should have suggestive names instead of case-XX.tact. The test file names should reflect the feature they test. If there is a lot of tests for one feature, we should consider grouping those into folders.

This will also help with merge conflicts, because name clashes between tests will be less likely for PRs that add different features.