Fix test_importlib.test_side_effect_import() (python#104840)

Wait until the thread spawn by the import completes to avoid dangling
threads. With this fix, the following command no longer fails:

./python -m test --fail-env-changed test_importlib -m test_side_effect_import -F -j20

Lib/test/test_importlib/test_threaded_import.py

@@ -238,7 +238,8 @@ def target():
         self.addCleanup(forget, TESTFN)
         self.addCleanup(rmtree, '__pycache__')
         importlib.invalidate_caches()
-        __import__(TESTFN)
+        with threading_helper.wait_threads_exit():
+            __import__(TESTFN)
         del sys.modules[TESTFN]
 
     def test_concurrent_futures_circular_import(self):