Always handle non-handled events before destoying root widget in tests.

This gets rid of  Tcl warnings when they are handled later when the root is
already destroyed.

Lib/tkinter/test/support.py

@@ -22,6 +22,7 @@ class AbstractTkTest:
 
     @classmethod
     def tearDownClass(cls):
+        cls.root.update_idletasks()
         cls.root.destroy()
         cls.root = None
         tkinter._default_root = None