testpackage addition: restart read on single(or multiple) mpi rank

[lassejsc]

Since the restart IO depends on the number of ranks here https://github.com/fmihpc/vlasiator/blob/dev/ioread.cpp#L1422 . By default the restarts are written with the --ntasks number of tasks so reading them with a single rank will use the else part of the code there. Not sure if this fully answers to #1163

Update:

Changed single_cell in test_definitions_small.sh to be ranks which sets the number of ranks to be used when running the test. Setting this to "half" as done in the new restart read tests will used half the $SLURM_NTASK ranks (floored division)

[ykempf]

Single rank in itself is already a good test but I think another rank count, but parallel, would be needed, say 8 ranks if the base case is run on 16, or so.