Lib/test/support/__init__.py::threading_cleanup() complains about dangling threads even if the reference in question would be cleaned up by the garbage collector.

This is not useful, esp. when the list of referrers to the "dangling" thread looks like this:

[<frame at 0x7fe830195768, file '/usr/lib/python3.7/threading.py', line 869, code run>, <frame at 0x7fe828000b38, file '/usr/lib/python3.7/threading.py', line 966, code _bootstrap_inner>, <frame at 0x7fe83018aac8, file '/usr/lib/python3.7/threading.py', line 889, code _bootstrap>]

Thus I propose to check, run gc, check again, and only *then* complain-and-wait. Hence the attached patch for your consideration.