Fix --walltime parsing for sub_test

When passing --walltime to the executable (for slurm and pbs) the string we
created was only valid for times under 100 minutes. After that it became
invalid. We never run into this in nightly testing, but I ran into it when
trying to see how long hpl release would take to finish for gasnet+mpi
Merge pull request #1045 from ronawho/fix-sub-test-walltime-parsing

Fix --walltime parsing for sub_test

[reviewed by @thomasvandoren]

When passing --walltime to the executable (for slurm and pbs) the string we
created was only valid for times under 100 minutes. After that it became
invalid. We never run into this in nightly testing, but I ran into it when
trying to see how long hpl release would take to finish for gasnet+mpi