bpo-31234: test_threaded_import: fix test_side_effect_import() (#3189)

* Don't leak the module into sys.modules
* Avoid dangling thread

Author: vstinner

Lib/test/test_threaded_import.py

@@ -230,7 +230,8 @@ def target():
                 import random
             t = threading.Thread(target=target)
             t.start()
-            t.join()"""
+            t.join()
+            t = None"""
         sys.path.insert(0, os.curdir)
         self.addCleanup(sys.path.remove, os.curdir)
         filename = TESTFN + ".py"
@@ -241,6 +242,7 @@ def target():
         self.addCleanup(rmtree, '__pycache__')
         importlib.invalidate_caches()
         __import__(TESTFN)
+        del sys.modules[TESTFN]
 
 
 @reap_threads