make sure not to run the restart test at the same time as the non-restarted test

normally, this should fine as it was before this patch because the
non-restarted tests produces more timesteps than the restarted one and
only the last file gets verified, but it used to be quite a hack and
one never knows, so it's wise to explicitly specify this as a
dependency.

Also, this change makes sure that ctest is aware of the number of
cores required by a test, which should lead to less contention...