Validate stats output by performance tests

In Jenkins perf-test-evict-fairness will only pass if exactly 200 pages are evicted

if [ $count -eq 200 ]; then 

    echo "eviction walk fairness pass" 

else

    echo "eviction walk fairness fail" exit 1 

fi 

but the Evergreen perf framework does not support assertion like this. A solution to pass/fail this test in Evergreen is required.

Definition of Done:
Evergreen can detect and report the pass/fail status of perf-test-evict-fairness.