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_ssl.py b/Lib/test/test_ssl.py
index 0093a49..4184665 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -408,8 +408,7 @@
         else:
             os.close(wfd)
             self.addCleanup(os.close, rfd)
-            _, status = os.waitpid(pid, 0)
-            self.assertEqual(status, 0)
+            support.wait_process(pid, exitcode=0)
 
             child_random = os.read(rfd, 16)
             self.assertEqual(len(child_random), 16)