[MERGE] Move skipped test detection to TestCase, and make reporting use an addSkip method as per testtools.

[Robert Collins]

This moves the detection that a test is skipping into TestCase rather
than being done after-the-fact by TestResult. It still supports the
protocol of calling addError with a TestSkipped, if the TestResult does
not have an addSkip method. I find this very useful with subunit - I can
accurately reload failing and erroring tests only (with 107 tests still
being false positives - and they are all missing features, but 107 is an
ok amount, for now).

-Rob
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