Add extend rollback handler tests to actions framework

Context

The actions framework fails to extend a job's rollback handler if the rollback handler is already defined in the base job. Use case: The LaunchInstances job never runs the chargeUser helper function in the custom SubT's rollback handler.

Change

This MR makes the Extend method replace the RollbackHandler if the extension given as an argument has a rollback handler defined.

Other information

Additional documentation