Opportunistically teach test decorators in framework/test_util.py to

return the decorated function's result, if any.  This makes the most
sense for decorators that run the decorated function no more than
once, since that makes it obvious which result to return.

This helps generalize them to sub-test or non-test scenarios where the
decorated function may return something valuable.

The change should be a no-op for tests, because they return None
anyway, and None was what the decorators were unconditionally
returning before this change.

PiperOrigin-RevId: 227528698