HLT: Fix and test magnetic field settings

In addition to the AtlasFieldSvc, set the magnet currents also on the new AtlasFieldMapCondAlg (ATR-21277). Once all clients have been migrated the former will be removed. Also implement an ART test to check magnet status switches during stop/start transitions. The test implements an ON->OFF->ON sequence and checks that all histograms are identical between the first and third run.

Also in this MR: delete some obsolete .trans files