[2.7] bpo-31160: Backport reap_children fixes from master to 2.7 (#3063)

[2.7] bpo-31160: Backport reap_children fixes from master to 2.7 (#3063)

* bpo-31160: regrtest now reaps child processes (#3044)

Add a post_test_cleanup() function which currently only calls
support.reap_children().
(cherry picked from commit e3510d74aacc477c30f42f2b941d69689bbc478e)

* bpo-31160: test_tempfile: Fix reap_children() warning (#3056)

TestRandomNameSequence.test_process_awareness() now calls
os.waitpid() to avoid leaking a zombie process.
(cherry picked from commit 6c8c2943d996b59a48d331f61f22cbe72933910e)