runner/resultgen: Make subtest result line finding more robust

On an assertion failure, the string "Subtest xyz failed" is
printed. Make sure we don't match that for SUBTEST_RESULT, or the
equivalent for dynamic subtests.

Parsing the results already explicitly searched for the proper result
line, the difference is when we delimit output up to "the next line of
interest".

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