Reducing AbstractClusteredWriteSkewTest.testSharedCounter duration

* looking at the logs in CI, it looks the test is taking too
long.
* both threads stop after incrementing the counter until 100

no JIRA for this. I only reduced the max number of counter value to avoid the failures with master JDK6: http://ci.infinispan.org/viewLog.html?buildId=3033&tab=buildResultsDiv&buildTypeId=bt2