annotationlib: Move ForwardRef tests to test_annotationlib (#132571)

I started with just moving ForwardRefTests to test_annotationlib,
but found that it contained a number of tests for no_type_check, which
I moved to a new class in test_typing, as well as a number of tests that
are more appropriately classified as tests for get_type_hints().

One test, test_forward_equality_namespace(), was somewhat accidentally
depending on a global class A in test_typing. I added a class A in the
annotationlib tests instead.

Also add a useful comment in annotationlib.