[v8r6] Move failoverSE definition to execute()

This was failing the tests, as in ModuleBase the data members are initialized in execute() which is not really optimal :( It is ugly but pragmatic… IMHO a review of the Module workflow should be reviewed (e.g add an initialize() method) as it is not good practice to initialize in execute()…

The BookkeepingReportSuccess test is failing, no idea why but seems unrelated to my changes as the file extension is missing from the LFNs. Could not check whether the problem was there before though. May be fixed in another MR?