bpo-40094: Add test.support.wait_process() (GH-19254)

Moreover, the following tests now check the child process exit code:

* test_os.PtyTests
* test_mailbox.test_lock_conflict()
* test_tempfile.test_process_awareness()
* test_uuid.testIssue8621()
* multiprocessing resource tracker tests

Lib/test/_test_multiprocessing.py

@@ -5124,7 +5124,7 @@ class TestResourceTracker(unittest.TestCase):
         pid = _resource_tracker._pid
         if pid is not None:
             os.kill(pid, signal.SIGKILL)
-            os.waitpid(pid, 0)
+            support.wait_process(pid, exitcode=-signal.SIGKILL)
         with warnings.catch_warnings():
             warnings.simplefilter("ignore")
             _resource_tracker.ensure_running()