Teach ISF new config test to run

Get ISF configuration to run a test without failing.

Many changes of private tools to public tools.

Add flag-based default decision for ConfigFlags.Sim.ISF.HITSMergingRequired (cc @jchapman )

Add Kernel_ATLFASTIIMTCfg

Get BaseSimulationSelector to accept an empty simulator, as needed for MT.

Numerous changes and suggestions are thanks to @bleban