Decrease number of iterations in TestMaxCachedBufferSize

Prior to the fix for JDK-8249772, progress was reported at 50% and 100% completion, and after that fix at 10% increments. Timeouts for the 50% increment case, which represents most of the reported failures, occurred at both 50% and 100%. The failures appear to be only on old machines and likely due to contention on the machine. Reducing the number of iterations in the test by 50% should hopefully eliminate the timeouts.