Fix race condition in test

If job submit time is right at a second boundary, the test could fail

TEST: 15.37
spawn /home/jette/SLURM/install_smd/bin/salloc --begin now+60 --deadline now+600 --time-min 10 sleep 1^M
salloc: error: Job submit/allocate failed: Requested time limit is invalid (missing or exceeds some limit)^M

FAILURE: batch not submitted with a deadline too short
test15.37 FAILURE

[2016-02-12T15:53:53.005] _valid_job_part: job's min_time greater than deadline (10 > 2016-02-12T16:03:52)
[2016-02-12T15:53:53.005] _slurm_rpc_allocate_resources: Requested time limit is invalid (missing or exceeds some limit)