Fix typos in TestThreadedProgressObservers tests

test_threaded_progress_on_begin and test_threaded_progress_on_end
were calling stop() on _patch rather than _mock_patch(), producing
an error on the subsequent _mock_patch.start() (since it was
already started, and hadn't been stopped). Corrected in this commit.

Fixes #1104

test/core/gen2/test_progress.py

@@ -112,7 +112,7 @@ def test_threaded_progress_on_begin(self):
 
         observer.on_begin([state_stack])
         self.assertTrue(_mock.called)
-        _mock.stop()
+        _mock_patch.stop()
 
         _mock = _mock_patch.start()
         state_stack1 = ProgressState("Test", 100, 100)
@@ -134,7 +134,7 @@ def test_threaded_progress_on_end(self):
 
         observer.on_end([state_stack])
         self.assertTrue(_mock.called)
-        _mock.stop()
+        _mock_patch.stop()
 
         _mock = _mock_patch.start()
         state_stack1 = ProgressState("Test", 100, 100)