Consolidate backend test runners

[giacomofiorin]

The different versions of the run_tests.sh script have diverged enough to make it difficult to track (and easy to forget) what conditions each version is running.

We have now the following conditions (list may be incomplete):

• Running the MD engine on multiple tasks
• Running over multiple threads
• Combination of the above two
• Multiple-replicas runs (for some interface tests only)
• Writing and reading text vs. binary state files

Now that the new GROMACS interface is merged in, it is a good time to start integrating them.