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
diff --git a/Lib/test/test_tracemalloc.py b/Lib/test/test_tracemalloc.py
index 7283d9c..635a9d3 100644
--- a/Lib/test/test_tracemalloc.py
+++ b/Lib/test/test_tracemalloc.py
@@ -314,10 +314,7 @@
             finally:
                 os._exit(exitcode)
         else:
-            pid2, status = os.waitpid(pid, 0)
-            self.assertTrue(os.WIFEXITED(status))
-            exitcode = os.WEXITSTATUS(status)
-            self.assertEqual(exitcode, 0)
+            support.wait_process(pid, exitcode=0)
 
 
 class TestSnapshot(unittest.TestCase):