[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)
diff --git a/Lib/test/test_tempfile.py b/Lib/test/test_tempfile.py
index 5c85cc9..5c111a2 100644
--- a/Lib/test/test_tempfile.py
+++ b/Lib/test/test_tempfile.py
@@ -141,12 +141,15 @@
         try:
             pid = os.fork()
             if not pid:
+                # child process
                 os.close(read_fd)
                 os.write(write_fd, next(self.r).encode("ascii"))
                 os.close(write_fd)
                 # bypass the normal exit handlers- leave those to
                 # the parent.
                 os._exit(0)
+
+            # parent process
             parent_value = next(self.r)
             child_value = os.read(read_fd, len(parent_value)).decode("ascii")
         finally:
@@ -157,6 +160,10 @@
                     os.kill(pid, signal.SIGKILL)
                 except EnvironmentError:
                     pass
+
+                # Read the process exit status to avoid zombie process
+                os.waitpid(pid, 0)
+
             os.close(read_fd)
             os.close(write_fd)
         self.assertNotEqual(child_value, parent_value)