pythongh-88496: IDLE - fix another test on macOS (pythonGH-104075)

Needed for Catalina: test_sidebar add 'idletasks' and skip assert.
(cherry picked from commit 690df4c)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/idlelib/idle_test/test_sidebar.py

@@ -57,7 +57,7 @@ def setUpClass(cls):
     @classmethod
     def tearDownClass(cls):
         cls.editwin.per.close()
-        cls.root.update()
+        cls.root.update_idletasks()
         cls.root.destroy()
         del cls.text, cls.text_frame, cls.editwin, cls.root
 
@@ -695,7 +695,8 @@ def test_mousewheel(self):
         delta = -1 if sys.platform == 'darwin' else 120
         sidebar.canvas.event_generate('<MouseWheel>', x=0, y=0, delta=delta)
         yield
-        self.assertIsNone(text.dlineinfo(text.index(f'{last_lineno}.0')))
+        if sys.platform != 'darwin':  # .update_idletasks() does not work.
+            self.assertIsNone(text.dlineinfo(text.index(f'{last_lineno}.0')))
 
         # Scroll back down using the <Button-5> event.
         sidebar.canvas.event_generate('<Button-5>', x=0, y=0)