bpo-45436: Fix tkinter tests with Tcl/Tk 8.6.11+ (GH-29077)

Since v8.6.11, a few configuration options seem to accept an empty value
where they did not previously; particularly the `type` of a `Menu`
widget, and the `compound` of any ttk widget with a label.  Providing an
explicit expected error message to `checkEnumParam` bypasses the check
of an empty value, which no longer raises `TclError`.

Lib/tkinter/test/test_tkinter/test_widgets.py

@@ -1241,8 +1241,11 @@ class MenuTest(AbstractWidgetTest, unittest.TestCase):
 
     def test_configure_type(self):
         widget = self.create()
-        self.checkEnumParam(widget, 'type',
-                'normal', 'tearoff', 'menubar')
+        self.checkEnumParam(
+            widget, 'type',
+            'normal', 'tearoff', 'menubar',
+            errmsg='bad type "{}": must be normal, tearoff, or menubar',
+            )
 
     def test_entryconfigure(self):
         m1 = self.create()