Separate simulator comparison tests

Separate simulator comparison tests

The simulator comparison tests ensure that legacy and modular
simulator code paths don't diverge while both exist. They are
currently part of the mdrun tests. Separating them has two
advantages:

* As the feature set of modular simulator grew, the run time
  of these tests became the largest contributor to the run time of
  mdrun-test. Keeping it separate makes it easy to keep the
  run time in check and to disable the tests if necessary.
* It is useful to run the simulator comparison tests on
  multiple ranks (at least locally). Not all other tests
  in mdrun-test support more than one rank. Having a separate
  binary simplifies running it with different parallelization schemes.