Lower rate by rate_granularity if final validation fails.
Lower rate by rate_granularity if final validation fails.

Normally just setting rate to (previousPassRate + rate)/2
is good enough, but sometimes the previous pass rate is actually
the same as rate, so the rate for the next test never gets reduced.
This enusres it does.

bmichalo/MoonGen,bmichalo/MoonGen,bmichalo/MoonGen,bmichalo/MoonGen