tests: add walltime in job description

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tests: lower walltime to 30 minutes for latency_probe on Mira

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tests: latency_probe does not need a sample

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>