GaudiConf: validateWithReference so slow tests do not complete

See comment DaVinci!733 (comment 5988992): in certain circumstances, make test will always hang and never complete. After some investigations this was traced to validateWithReference. The workaround for this problem is to throw away the reference, at which point the test will complete and fail, and then move the new test file on top of the reference, and use git diff to compare the files and make a decision on whether the changes are expected.

Now, figuring out the above has cost me quite some time, and surely this will happen and has happened to others as well, so validateWithReference should be adapted to not go into the moral equivalent of an infinite loop -- even just 'giving up' would be preferable...