Fix testing with large numbers of MPI ranks

Without this change, on (say) simple/morse, gmxtest.pl -np 64 would
fail, then try -ntmpi 8, then try serial, then maybe have a syntax
error. Now, it retries with -np 8 like it should.

Change-Id: I44b2ba51a9f22fcb3e1453d363b20fabb88133f2