runner/resultgen: Provide output when test output is completely empty

If a machine is hard-hanging or otherwise rebooted at the correct
time, intermediary output files get created but nothing ever gets
written to them. That yields results that are completely empty and
hard to categorize or even sometimes detect automatically. Handle this
corner case explicitly with a custom text explaining what might have
happened to prod result analysis towards fixing the real issue instead
of wondering if test result processing is faulty.

The race for getting empty files is easier to hit than it seems. The
files get created by the runner before calling exec(), and there's
plenty of time to hit a really hard crash.

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