ktest: add ability to skip during BISECT_MANUAL

When doing a manual bisect, a build can fail or a test can be inconclusive.
In these cases it would be helpful to be able to skip the test entirely.

Link: http://lkml.kernel.org/r/1409164021-2136-1-git-send-email-chris.j.arges@canonical.com
Reviewed-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>