Improve stability of ContinuationIsExact tests

The default nstlist value in tests is set to 8. This is appropriate for NormalIntegrators/MdrunNoAppendContinuationIsExact tests, which do the restart after 8 steps, so the "full" trajectory and the "restarted" trajectory update their lists at the same time. However, pairlist tuning often increases the nstlist value, which might affect the "full" run.

Here, we force the nstlist to be half the nsteps for the "full" run by setting it via CLI option.