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.
I did it this way on purpose. I thought it was logically clearer to have things grouped into a "reading" section and a "writing" section.