test: reset test_subtest_known_broken_ after each success/failure.

This means that test_subtest_known_broken needs to be called before
every known broken subtest, which is no different than what is
documented for the test_begin_subtest case.

The assumption is that every test ends up calling either skipping,
calling test_ok_ or test_failure_ and and the latter in turn delegate
to the known_broken versions in the case where
test_subtest_known_broken_ is set.