POC for multi node test

This is currently a mess. The command line arguments are hacked in. The node test runner project references Akka.Cluster.Tests when it shouldn't, for convenience.

There is some useful stuff here around xunit test discovery though.

I was debugging the NodeTestRunner on its own with the following command line 

Akka.Cluster.Tests.dll Akka.Cluster.Tests.MultiNode.JoinInProgressMultiJvmNode1 AClusterNodeMustSendHeartbeatsImmediatelyWhenJoiningToAvoidFalseFailureDetectionDueToDelayedGossip 1 localhost localhost 0

This was useful to pick up a number of bugs with the multi node test framework.

Once that works. The multi node runner should pick up and run tests in multiple processes. Although again this could be a lot more refined. 

Hope it is of some use.