ci/tests: handle known issues as expected failures

[jacobpake]

Individual tests listed in .known-issues no longer effect the overall test suite failure state.

These tests are still run and their errors are still emitted, along with the associated issue number, e.g.

frontend/tests/cases/positive/indexed/infer-kinds.gr:                                         FAIL (expected: #299)
      Exception: Type checking failed:
      Kind mismatch: frontend/tests/cases/positive/indexed/infer-kinds.gr:10:1:
      Expected kind `Type 2` for type `Coeffect` but actual kind is `Type`
      CallStack (from HasCallStack):
        error, called at interpreter/tests/Golden.hs:152:21 in main:Main (expected failure)

This allows us to have meaningful CI status checks without removing the tests. Ideally, we should be able to remove these changes once all issues are resolved.

[dorchard]

That's cool - thanks for this! We can fix those remaining ones hopefully at the next hackathon!