[issue21337] Add tests for Tix

[Terry J. Reedy]

Terry J. Reedy added the comment:

A minimal test would be that the one in the doc.

from tkinter import tix
root = tix.Tk()
root.tk.eval('package require Tix')

This passes on my 3.4.2 win7.  I believe the first line should work on any system with _tkinter, whereas 
https://stackoverflow.com/questions/27751923/tix-widgets-installation-issue

reports failure of the second line on his Mac with ...
  self.tk.eval('package require Tix')
_tkinter.TclError: can't find package Tix

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nosy: +terry.reedy

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