Make job_manage available cpu time test more robust

The job_manage_avail_time test checks that gs2 exits when the time elapsed exceeds avail_cpu_time. This test used to check that gs2 exited “reasonably soon” after the available time ran out, but sporadically failed as “reasonably soon” varies from machine to machine.

In this PR, the test no longer times how long it takes to exit; instead it checks:

1. that time elapsed exceeds avail_cpu_time; and

2. that fewer than `nstep` timesteps have been completed