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

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`.