new test: sticky sessions

[Geal]

• set up an application with two backends, and sticky sessions (possibly with a specific sticky id)
• the two backends send their id in the response
• the client sends a request (no keep-alive)
• we check that we got a sticky session cookie, and we store the id of the backend that answered
• the client sends a second request, with the session cookie
• we check that we got a response from the same backend
• we remove all the backends and set up new ones
• the client sends a third request, with the session cookie
• we check that we got a response from a different backend, and a new session cookie

[NotBad4U]

Done in 20cba37