[issue18604] Consolidate gui available checks in test.support

[Roundup Robot]

Roundup Robot added the comment:

New changeset 7ecb6e4b1077 by Ned Deily in branch '3.4':
Issue #18604: Skip the Tk instantiation test on OS X because it can
http://hg.python.org/cpython/rev/7ecb6e4b1077

New changeset 7f6d9990a9b1 by Ned Deily in branch 'default':
Issue #18604: merge from 3.4
http://hg.python.org/cpython/rev/7f6d9990a9b1

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<http://bugs.python.org/issue18604>
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