Add a test case for LumiList just for one task, make it work

Fixes #5314 

You can now specify LumiList in a Task section and its set. Then the task iterator which sets it for every task in a WF is told to ignore it if it's already there. @amaltaro , please review.
Eric, if I correctly understood, if we have already set a lumiMask then it will remain for the whole request (since setLumiMask will only return with no changes). For instance, in case there is a second task with different InputDset and LumiList, it won't override the lumiMask, but instead it will keep the lumiMask from the previous task. Is that correct?