bpo-33613: Fix test_semaphore_tracker signal tests when using -Werror (GH-9778)

Tests involving sending signals to the semaphore_tracker will not fail anymore due to
the fact that running the test suite with -Werror propagates warnings as errors.

Fix a missing assertion when the semaphore_tracker is expected to die.

Lib/test/_test_multiprocessing.py

@@ -4548,7 +4548,8 @@ class TestSemaphoreTracker(unittest.TestCase):
         if pid is not None:
             os.kill(pid, signal.SIGKILL)
             os.waitpid(pid, 0)
-        with warnings.catch_warnings(record=True) as all_warn:
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore")
             _semaphore_tracker.ensure_running()
         pid = _semaphore_tracker._pid
 
@@ -4557,6 +4558,7 @@ class TestSemaphoreTracker(unittest.TestCase):
 
         ctx = multiprocessing.get_context("spawn")
         with warnings.catch_warnings(record=True) as all_warn:
+            warnings.simplefilter("always")
             sem = ctx.Semaphore()
             sem.acquire()
             sem.release()
@@ -4569,7 +4571,7 @@ class TestSemaphoreTracker(unittest.TestCase):
             if should_die:
                 self.assertEqual(len(all_warn), 1)
                 the_warn = all_warn[0]
-                issubclass(the_warn.category, UserWarning)
+                self.assertTrue(issubclass(the_warn.category, UserWarning))
                 self.assertTrue("semaphore_tracker: process died"
                                 in str(the_warn.message))
             else: