Issue #10734: Fix and re-enable test_ttk test_heading_callback.

Lib/tkinter/test/test_ttk/test_widgets.py

@@ -1369,12 +1369,10 @@ class TreeviewTest(AbstractWidgetTest, unittest.TestCase):
         self.assertRaises(tkinter.TclError, self.tv.heading, '#0',
             anchor=1)
 
-    # XXX skipping for now; should be fixed to work with newer ttk
-    @unittest.skip("skipping pending resolution of Issue #10734")
     def test_heading_callback(self):
         def simulate_heading_click(x, y):
             support.simulate_mouse_click(self.tv, x, y)
-            self.tv.update_idletasks()
+            self.tv.update()
 
         success = [] # no success for now