typing, annotationlib: clean tests

- Add @cpython_only decorator to lazy import tests
- Rename reference to SOURCE format
- Always two newlines between test case classes
- Merge two classes of ForwardRef tests
- Use get_annotations instead of annotationlib.get_annotations
- Format test_annotationlib with Black (not expecting that we'll keep this up
  but it's close to Black-formatted right now)

Author: JelleZijlstra