Make test 924 more consistent.

Test 924 was flaking due to relying on timing between threads and
short time windows. This lengthens the time windows and tries to test
for the desired state more directly

Test: stress --cpu 100 &;
      while ./test/run-test --host 924; do; done
Bug: 62117833

Change-Id: I5119255afbd793ae92358250281b77c783381b2d