AbstractClusteredWriteSkewTest can fail randomly still

Caffeine has some interesting mechanics to how it controls eviction. These can cause this test to fail with "AssertionError: The key was not evicted after 10 inserts".

This is explained at https://github.com/infinispan/infinispan/pull/4578#discussion_r89352984. We should change the test so it will consistently work, be it by changing the keys or by some other manner.