Same hang in 2.6.0 NUnit GUI with or without the change. I can try console later today if you need me to.

To be more precise, the "hang" actually takes 10 minutes or so, after which it comes back with this exception:

System.Net.Sockets.SocketException : A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond

Server stack trace:

Server stack trace:
at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
at System.Runtime.Remoting.Channels.SocketStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.Runtime.Remoting.Channels.SocketHandler.ReadFromSocket(Byte[] buffer, Int32 offset, Int32 count)
at System.Runtime.Remoting.Channels.SocketHandler.Read(Byte[] buffer, Int32 offset, Int32 count)
at System.Runtime.Remoting.Channels.SocketHandler.ReadAndMatchFourBytes(Byte[] buffer)
at System.Runtime.Remoting.Channels.Tcp.TcpSocketHandler.ReadVersionAndOperation(UInt16& operation)
at System.Runtime.Remoting.Channels.Tcp.TcpClientTransportSink.ProcessMessage(IMessage msg, ITransportHeaders requestHeaders, Stream requestStream, ITransportHeaders& responseHeaders, Stream& responseStream)
at System.Runtime.Remoting.Channels.BinaryClientFormatterSink.SyncProcessMessage(IMessage msg)

Exception rethrown at [0]:
at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
at Microsoft.Hpc.Scheduler.Store.ISchedulerStoreInternal.Register(String clientSource, String userName, ConnectionRole role, Version clientVersion, ConnectionToken& token, UserPrivilege& privilege, Version& serverVersion, Dictionary`2& serverProps)
at Microsoft.Hpc.Scheduler.Store.StoreServer.RegisterWithServer()
at Microsoft.Hpc.Scheduler.Store.StoreServer._Connect()
at Microsoft.Hpc.Scheduler.Store.StoreServer.Connect(String server, Int32 port, Boolean serviceAsClient, ServiceAsClientIdentityProvider identityProvider, String serviceAsClientUser, String serviceAsClientPass)
at Microsoft.Hpc.Scheduler.Store.SchedulerStoreSvc..ctor(String server, Int32 port, Boolean serviceAsClient, ServiceAsClientIdentityProvider provider, String serviceAsClientUser, String serviceAsClientPass, Boolean overHttp)
at Microsoft.Hpc.Scheduler.Store.SchedulerStoreSvc.RemoteConnect(String server, Int32 port, Boolean serviceAsClient, ServiceAsClientIdentityProvider provider, String userName, String password, Boolean overHttp)
at Microsoft.Hpc.Scheduler.Store.SchedulerStoreSvc.RemoteConnect(String server, Int32 port)
at Microsoft.Hpc.Scheduler.Store.SchedulerStore.Connect(String server)
at Microsoft.Hpc.Scheduler.Scheduler.Connect(String cluster)
at TestHPC.TestHpc.AllCoresPresent(String headNode, Int32 expectedCores) in C:\Obfuscated\Path\TestHPC\TestHPC.cs:line 18

So it seems there are a couple of ways the test can fail.

You need to install the HPC cluster OS on a separate machine:
http://www.microsoft.com/en-us/download/details.aspx?id=13769

Apply this service pack:
http://www.microsoft.com/en-us/download/details.aspx?id=28017

On your dev machine, install the HPC Pack 2008 R2 SDK (http://www.microsoft.com/en-us/download/details.aspx?id=28016) and client utilities (http://www.microsoft.com/en-us/download/details.aspx?id=17017). These have the dlls I'm referencing:
Microsoft.Hpc.Scheduler.dll
Microsoft.Hpc.Scheduler.Properties.dll
Microsoft.Hpc.Scheduler.Session.dll
Microsoft.Hpc.Scheduler.Store.dll

Make sure you use the same version of the dlls (3.3.3950). Hopefully I gave you the right links, but, if not, you can browse related downloads and find the right version. Essentially, 3.3.3950 corresponds to service pack 3 of HPC.