Added unit tests and acceptance tests for gateway

- Added unit tests for Gateway
- Added gateway related acceptance tests.

**NOTE:** 
Adding the `NServiceBus.AcceptanceTests.Sources` nuget package, brings in various other tests that are not related to gateway. However, this is needed because some core classes that are needed for the acceptance test framework such as `NServiceBusAcceptanceTest` class and `DefaultServer` etc are defined here.