Use more appropriate asserts in re tests.

--HG--
branch : 2.7
Now re tests work again on Unicode-disabled build.
Simplified existing detections of Unicode-disabled build.

--HG--
branch : 2.7
Issue #22384: An exception in Tkinter callback no longer crashes the program
when it is run with pythonw.exe.

Documented that Tk.report_callback_exception() is purposed to be overriden in
applications.

--HG--
branch : 2.7