[issue18604] Consolidate gui available checks in test.support

[R. David Murray]

R. David Murray added the comment:

I think it is fine to backport this, since it only affects tests, and allows the backported idle tests to be consistent.

For 3/4, I agree that requires should skip always if the GUI infrastructure is not available.  The "don't skip if main" logic is appropriate only for deciding whether or not to run the tests when the test suite is invoked directly (as opposed to through regrtest), not for deciding whether or not to run them when the actual resource is being checked.

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nosy: +r.david.murray

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