XD-794 Add integration tests for SpEL/Groovy based routing

General comments:

1. should there be an abstraction in place to retrieve messages from a "named channel" programmatically? Right now I don't think there is an implementation agnostic way of doing this (such as `receiveMessage()`, `queueSize()`). I think this could be quite useful for integration tests of streams. E.g. to do more focussed tests without resorting to "temp-files" and non-essential sinks or sources etc. - e.g. `:routeit > router --expression=payload.contains('a')?':foo':':bar'`
2. when named channels are created dynamically - should there be a hook to get notified when it is up and running? I had to resort to Thread.sleep

Point **1)** is the reason I put the tests under `SingleNodeStreamDeploymentIntegrationTests`.