bpo-36674: Honour the skipping decorators in TestCase.debug() (GH-28446)

unittest.TestCase.debug() raises now a SkipTest if the class or
the test method are decorated with the skipping decorator.

Previously it only raised a SkipTest if the test method was decorated
with other decorator in addition to the skipping decorator, or
if SkipTest was explicitly raised in the test or setup methods.