Split test jobs to avoid duplicate work

[casella]

Currently we have some test jobs that unnecessarily carry out duplicate work:

• the master-fmi and the master-fmi-fmpy jobs each build the FMUs before running them, but that's not necessary, the FMUs could just be built once and then ran twice, once with OMSimulator and once with FMPy
• the gbode task currrently runs once with the default parameters - in the future, we could try different gbode settings (e.g. different algorithms, possibly including multi-rate) for which there is no need to re-compile the simulation code.

We should probably split some of the test jobs to avoid this issue.

[casella]

For the FMI jobs, it seems that the majority of time is spent building the code, so splitting the test job would lead to a saving of almost 50% of the time.