Add art-result when references are missing

    Currently we don't notice when there are missing reference files for the Trigger ART tests.
    This change will return an error via art-result if the reference is missing

	modified:   Trigger/TrigValidation/TriggerTest/test/exec_art_triggertest_post.sh


Former-commit-id: 8330d17cde7a4bf4e8cb7b1ef4f9fefb683e14e6