new test: remove backend between requests

[Geal]

• set up an application with one backend
• start a server for that backend, and another server
• those servers indicate their id in the response
• the client sends a request (in keep alive)
• check that we receive a response, and that it comes from the first backend
• change the application's configuration in sozu to remove the first backend and replace it with the second one
• the client sends a request
• check that we receive a response, and that it comes from the second backend

[NotBad4U]

Done in c75d14b

[Geal]

how can we be sure that the requests are done in keep-alive? It is not enough to pass the Connection: keep-alive header (and is probably redundant, since HTTP 1.1 is keep-alive by default), we have to be sure that the same TCP connection is used.