* 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 4baca1b0f7)
* 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 e3510d74aa)
* bpo-31160: test_builtin: don't check waitpid() status (#3050)
(cherry picked from commit 3ca9f50f96)
* 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 6c8c2943d9)
* Remove runtest_ns(): pass directly ns to runtest().
* Create also Regrtest.rerun_failed_tests() method.
* Inline again Regrtest.run_test(): it's no more justified to have a method
Move the code to run tests in multiple processes using threading and subprocess
to a new submodule.
Move also slave_runner() (renamed to run_tests_slave()) and
run_test_in_subprocess() (renamed to run_tests_in_subprocess()) there.
