Add test specifically for testing ns-type = simple

Also infrastructure for letting gmxtest.pl adapt to the presence of
different numbers of GPUs. If a test case can only run with one rank,
then there can only be one GPU id supplied, whereas Jenkins tests a
two-GPU system. So, gmxtest.pl needs a more complex interface for
handling mdrun -gpu_id than just adding flags to gmxtest.pl -mdparam.
Just using GMX_GPU_ID didn't work.

Change-Id: Ifca0ac5a69b9039a4c063582279efa94cfd7f1b6