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_builtin.py b/Lib/test/test_builtin.py
index 1e9012e..86e5f1d 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -25,6 +25,7 @@
 from textwrap import dedent
 from types import AsyncGeneratorType, FunctionType
 from operator import neg
+from test import support
 from test.support import (
     EnvironmentVarGuard, TESTFN, check_warnings, swap_attr, unlink,
     maybe_get_event_loop_policy)
@@ -1890,7 +1891,7 @@
         os.close(fd)
 
         # Wait until the child process completes
-        os.waitpid(pid, 0)
+        support.wait_process(pid, exitcode=0)
 
         return lines