xAODDataSource Unit Test Fix, master branch (2021.07.12.)

This is a tiny change to make this unit test work in Debug builds. Which currently it doesn't. See:

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/ardoc_web_areamaster64BC7G8AthenaDbg/ARDOC_TestLog_2021-07-11T2101/Control_xAODDataSource___xAODDataSourceConf__xAODDataSourceTest__m.html

I was looking at generalising the code from

https://gitlab.cern.ch/atlas/athena/-/blob/master/Control/xAODRootAccess/python/GenerateDVIterators.py#L36-52

, so that the same "dictionary loading" code could be called by this test, and by that code. But that somehow just didn't want to work out. 😕 (I'm really not sure why...) So in the end I settled for this simple, but not general solution.