Do not report comparison failure for master-vs-21.9 run4 IDART tests

As brought up by @nstyles in https://its.cern.ch/jira/browse/ATLASRECTS-7093, the master-vs-21.9 run 4 IDART tests are unlikely to ever match perfectly, thus making dcube's warn/fail return codes not useful. DCube returns -1 (which wraps to 255 in the shell) when it encounters an actual software issue, so this MR adds a one-liner to clip the return code passed via art-result to 0 if they are not 255, and only for the master-vs-21p9 invocation.

cc @keli