tests: add exit case "KNOWN FAILED"

Now we can mark tests as known to be broken without bothering
users with exit failure. In the build log this "KNOWN FAILED"
may be interpreted as TODO ;)

The main advantage of "known-fail" instead of just using "skip"
is that we will still find the test diff.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>