Add integration tests

[yanns]

We should test the proxy as a black box.

Example of test:

• start the proxy with 2 backends
• kill one of the backend
• during all the process, checks that all requests to the proxy are successful

Another example of test:

• start the proxy with 1 backend
• add one backend with the REST API
• remove the first backend with the REST API
• during all the process, checks that all requests to the proxy are successful

Implementation: we could implement those tests with docker containers

[maniankara]

@yanns For this, my plan is to first get the possibility to run multiple test-servers. Ofc this is also helpful for the users to test this. Let me know how do you feel about this?

[yanns]

@maniankara OK with that. We can also simply use docker images on nginx that always return 200