It's a bit "hidden".

In these tests we prepare the server by adding behaviours. A behaviour consists of a request and a response. It means something like "if a get this request, i will answer this response". 

So, when we invoke the createOpenFlowForwardingRule(forwardingRule), our floodlight driver makes a call to our mockServer. The server is the one checking if the request matches one of the behavious we prepared, in terms of HttpMethod, Body and Content Type. If it matches, the server answers with the response of the matching behaviour.

So, just that the server doesn't response with a bad request, and checking that the Forwarding rule is in the switch model (checked by calling the getForwardingRules method) asserts that the driver sends the expected message.