[issue22236] Do not use _default_root in Tkinter tests

[Ned Deily]

Ned Deily added the comment:

See Issue22770 for details of a potential Tk crash that can occur on OS X when running tests as a result of these changes and for a workaround.

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nosy: +ned.deily

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22236>
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