[issue18604] Consolidate gui available checks in test.support

[Roundup Robot]

Roundup Robot added the comment:

New changeset fef11a65a5e5 by Ned Deily in branch '2.7':
Issue #18604: Skip the Tk instantiation test on OS X because it can
http://hg.python.org/cpython/rev/fef11a65a5e5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18604>
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