[3.13] gh-155732: Join the terminated process in test_repr_lock (GH-155734) (GH-155768)

The process was terminated, but not joined, so it was still reported as
a dangling process.
(cherry picked from commit 3c414a5)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

Lib/test/_test_multiprocessing.py

@@ -1493,6 +1493,7 @@ def test_repr_lock(self):
         event.wait()
         self.assertEqual(f'<Lock(owner=SomeOtherProcess)>', repr(lock))
         p.terminate()
+        p.join()
 
     def test_lock(self):
         lock = self.Lock()