New keyword ``TEST-KNOWN-FAILURE`` to mark tests that are currently
known to fail.

``@TEST-KNOWN-FAILURE``
   Marks a test as known to currently fail. This only changes BTest's
   output, which upon failure will indicate that that is expected; it
   won't change the test's processing otherwise. The keyword doesn't
   take any arguments but one could add a descriptive text, as in ::

    .. @TEST-KNOWN-FAILURE: We know this fails because ....