Deprecate / disable AbstractSubsystemBaseTest 'subsystem template' handling

The testSchemaOfSubsystemTemplates and getSubsystemTemplatePaths methods should be deprecated, and testSchemaOfSubsystemTemplates turned into something that just throws AssumptionViolationException.

A possible thing to do as throwaway work on this would be to make testSchemaOfSubsystemTemplates throw an exception and add a new @Test method that calls getSubsystemTemplatePaths and fails if it returns a non-empty String[]. Let CI have at it and use that to identify all the modules that still have this subsystem template stuff sitting around, so we can create issues to clean that out. We wouldn't merge this bit.