The main issue was that the tests specified `-library-level=api` in
`swift-frontend` invocations instead of `-library-level api`. The former seems
to have no effect and therefore expected diagnostics were not being emitted.
Additional clean up:
- Make every test require `OS=macosx` for ease of local testing.
- Avoid unnecessarily specifying target arches (this makes tests slow).
- Add missing `FileCheck` invocation.
- Add some missing test coverage.
Resolves rdar://91325474
