ISPN-4323 ConditionalOperationsConcurrentWriteSkewTest fails randomly

* Changed 500 invocation tests to stress as they can last for minutes
* Moved more simple test to separate file and still functional

https://issues.jboss.org/browse/ISPN-4323
That is on the original Test class when I copied it over, TBH I had removed it for a sec but kept it just because the other on had it.

https://github.com/infinispan/infinispan/blob/master/core/src/test/java/org/infinispan/api/ConditionalOperationsConcurrentTest.java#L37

I am fine with removing if you want.