Allow defining custom MPI launchers for systems

There are some systems I have access to where it’d be beneficial to use custom MPI launchers, different from the standard ones. Latest case I ran into is https://www.dur.ac.uk/icc/cosma/support/rockport/, where mpirun should be called as mpirun $RP_OPENMPI_ARGS, where $RP_OPENMPI_ARGS is an environment variable set by a module, which has to be loaded when running the tests (for example by setting systems.partitions.modules).

I may have time to work on this, but I need some guidance, especially in terms of what the API should be. In general, I want this to be system-specific (so to be customised in the system configuration), I don’t want to entangle tests with system-specific details.

[giordano]

Yes, I also wasn’t expecting it to work but it seems it does, and that’s perfect for my use case. The important seems to be to register the launcher before using it. An unregistered launcher still throws a useful error message:

reframe: failed to initialize runtime: no such launcher: 'unregistered_launcher'

[vkarak]

I have also been able to define a new launcher in the configuration. So we will relax the configuration schema and add a small tutorial on how to define a new custom launcher.