ktest: Add BISECT_SKIP

If a during a git bisect, ktest fails on something other than
what it is testing (if BISECT_TYPE is test but it fails on build),
if BISECT_SKIP is set, then it will do a "git bisect skip" instead
of just failing the bisect and letting the user find a good commit
to test.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>