gh-101267: make the regression test deterministic and self-checking

* Block the workers on a never-set Event (via the harness create_event())
  instead of submitting time.sleep(3): the sleep could finish before the
  worker is terminated on a loaded machine, completing the future and
  raising no BrokenProcessPool.
* Use try/except/else with self.fail() rather than seeding count = None,
  so a missing exception is reported clearly. assertRaises() can't be used
  here because it strips the traceback the test needs to inspect.

Author: gpshead

Lib/test/test_concurrent_futures/test_process_pool.py

@@ -63,25 +63,30 @@ def test_killed_child(self):
         # Submitting other jobs fails as well.
         self.assertRaises(BrokenProcessPool, self.executor.submit, pow, 2, 8)
 
+    @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
     def test_broken_process_pool_traceback(self):
         # When a child process is abruptly terminated, the whole pool gets
         # "broken", and a BrokenProcessPool exception should be created
         # for each future instead of sharing one exception among all futures.
-        futures = [self.executor.submit(time.sleep, 3) for _ in range(3)]
-        # Get one of the processes, and terminate (kill) it.
+        event = self.create_event()
+        futures = [self.executor.submit(event.wait) for _ in range(3)]
         p = next(iter(self.executor._processes.values()))
         p.terminate()
         for fut in futures:
-            count = None
+            # Don't use assertRaises(): it clears the traceback off exc.
             try:
                 fut.result()
-            except BrokenProcessPool as e:
-                count = sum(
-                    1
-                    for frame_summary in traceback.extract_tb(e.__traceback__)
-                    if frame_summary.filename == __file__
-                )
+            except BrokenProcessPool as exc:
+                tb = exc.__traceback__
+            else:
+                self.fail("BrokenProcessPool not raised")
+            count = sum(
+                1
+                for frame_summary in traceback.extract_tb(tb)
+                if frame_summary.filename == __file__
+            )
             # This code file should appear exactly once in the traceback.
+            # A shared exception would accumulate a frame per result() call.
             self.assertEqual(count, 1)
 
     @warnings_helper.ignore_fork_in_thread_deprecation_warnings()