The NonBlockingLocalTransportSender is used to talk to a proxy service from another proxy service. Since the nhttp transport is written in a non blocking manner, NonBlockingLocalTransport will work against nhttp transport. Since, we are using this TransportSender to talk between proxy services, it's difficult to come up with a test case (test client) for this particular usecase.
Since we are using NonBlockingLocalTransportSender to handle a special case (in which we are working against a non-blocking transport), the LocalTransportTest test will fail for NonBlockingLocalTransportSender. Please refer Axis-dev for the discussion we had on  where we discussed this in-detail.
The main reason for doing this local-transport related change at axis2 level is to remove the code duplication. Otherwise if we move the NonBlockingLocalTransport related logic to Synapse, we might have to duplicate the same local-transport related code there and improve it.
If we dont include this improvement into Axis2-local transport, then we have to improve the local transport in such a way that a user should be able to extended the local transport implementation and write a custom implementation. That will help us to move the Synapse specific local transport to Synapse itself.
 - http://markmail.org/thread/qigfb5asu22qestz