[3.6] bpo-31160: Backport reap_children() fixes from master to 3.6 (#3060)

[3.6] bpo-31160: Backport reap_children() fixes from master to 3.6 (#3060)

* bpo-31160: Fix test_builtin for zombie process (#3043)

PtyTests.run_child() now calls os.waitpid() to read the exit status
of the child process to avoid creating zombie process and leaking
processes in the background.
(cherry picked from commit 4baca1b0f7325032598cd38e7ceffc79b616d255)

* 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_builtin: don't check waitpid() status (#3050)

(cherry picked from commit 3ca9f50f96cfa5c1b7aa56639042531b57f07fbb)

* 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)