fix skip test if file not present
the skip decorator was not effective since in Python TFile::Open
raises an exception if the file is not present. The test was failing if the file was not present, while it should have been skipped.
Also, move to a default test file (on cvmfs).
Closes ATLASRECTS-7925
Labelling as urgent since this is breaking nightlies since the file was deleted by mistake.
Edited by Ruggero Turra