bpo-32294: Fix multiprocessing test_semaphore_tracker() (GH-4885) (#4887)

Run the child process with -E option to ignore the PYTHONWARNINGS
environment variable.
(cherry picked from commit 9402c8367b)

Lib/test/_test_multiprocessing.py

@@ -4146,7 +4146,7 @@ class TestSemaphoreTracker(unittest.TestCase):
         '''
         r, w = os.pipe()
         p = subprocess.Popen([sys.executable,
-                             '-c', cmd % (w, w)],
+                             '-E', '-c', cmd % (w, w)],
                              pass_fds=[w],
                              stderr=subprocess.PIPE)
         os.close(w)