Fix broken test for MutableSet.pop() (pythonGH-25209)

Changes the test to not assert concrete result of pop, but just that it
was an item from the set, and that the set shrunk by one.
(cherry picked from commit 453074c)

Co-authored-by: Stepan Sindelar <me@stevesindelar.cz>

Lib/test/test_collections.py

@@ -1502,8 +1502,12 @@ def discard(self,v):
                 return result
             def __repr__(self):
                 return "MySet(%s)" % repr(list(self))
-        s = MySet([5,43,2,1])
-        self.assertEqual(s.pop(), 1)
+        items = [5,43,2,1]
+        s = MySet(items)
+        r = s.pop()
+        self.assertEquals(len(s), len(items) - 1)
+        self.assertNotIn(r, s)
+        self.assertIn(r, items)
 
     def test_issue8750(self):
         empty = WithSet()