TestTools: make missing reference file a failure
Treat a missing unit test reference file as test failure. If no reference file is needed the unit test should define an explicit post-procssing script, e.g.:
atlas_add_test( ... POST_EXEC_SCRIPT nopost.sh )
This closes the loop-hole where the developer did provide a reference file but due to e.g. a typo in its name it is not being found.