Avoid thread leaks for thread sanitizer

[colesbury]

We currently have a suppression for thread leaks in thread sanitizer:

cpython/Tools/tsan/suppressions.txt

Lines 4 to 5 in c2202a7

	# https://gist.github.com/mpage/daaf32b39180c1989572957b943eb665
	thread:pthread_create

It'd be nice to remove this suppression. There are a two sources of thread leaks that I know:

1. It configure.ac, one of the checks doesn't pthread_detach or pthread_join. This is easy to fix
2. Some of our multiprocessing tests leak threads. We should probably just skip these tests when running under TSan.

Linked PRs

• gh-143121: Avoid thread leak in configure #143122
• gh-143121: Skip test that leak threads under TSan #143125