runner: Abort the run when test exits with IGT_EXIT_ABORT

Now that the IGT tests have a mechanism for signaling broken testing
conditions we can stop the run on the first test that has noticed it,
and possibly has triggered that state.

Traditionally run would have continued with that test failing and the
side effects would trickle down into the other tests causing a lot of
skip/fails.

v2: extra explanations, small cleanup (Petri)

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>