limits_test: validate CPU time used, not real time

RLIMIT_CPU applies to CPU time, not real (wall-clock) time.
This test failed in AWS, where the real time was 5-7 seconds.

Sum the user and system CPU time used, and validate that.

While I'm here, don't bother specifying -s exit:0 or -e empty,
since those are checked by default.

MFC after: 1 week
Sponsored by: Dell EMC Isilon