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.
(cherry picked from commit dea59cf88adf5d20812edda330e085a4695baba4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>