try_run: Add option to specify how test binary is launched

Per a thread on the Discourse, I have a case where it would be useful to use try_run but with an MPI application. As far as I know, there is no way to do this (or perhaps no easy way). @ben.boeckel thought having a LAUNCHER option might be one way to do so. Probably better than something MPI-specific since I suppose some systems rely on other launchers. Perhaps we'd need a LAUNCHER and LAUNCHER_ARGS a la ARGS so we can pass specific flags to the launcher?

(Note: Per that thread, I still think I might be using try_run incorrectly as it just...never seemed to run anything, but had it worked and without mpirun -np 1, it should have been a success.)