ISPN-2714 TopologyAwareTwoNodesMapReduceTest.testInvokeMapperCancellation test fails randomly

SimpleTwoNodesMapReduceTest.testInvokeMapperCancellation used a static
counter, replace it with a static map of barriers, keyed with the name of
the actual test class. That way, subclasses like
TopologyAwareTwoNodesMapReduceTest will each use their own barrier, and
they will be able to run in parallel.