Optimize FMI testing

[casella]

As I understand, we currently have two FMI test jobs. One uses OMSimulator, the other one uses FMPy to run the actual simulations. Unfortunately, both build the FMUs, which are rigorously the same. We should make this just one task, where the FMUs are built only once and run with many FMI simulation tools. That should be a parameter of the job. I would currently enable FMPy by default, and possibly use OMSimulator and PyFMI.