Add waitForHttp

[michivi]

This PR adds the waitForHttp which waits until the given container port returns one of the acceptable status codes.

This is required as in some cases, waitUntilMappedPortReachable is not enough: the containerized Web container is listening to requests but is actually still not ready to take them.

Some notes regarding this implementation:

• It is limited to GET requests. It may be interesting to be more flexible on the request in the future.
• A fresh new Manager is created for each port to be tested, which may be kind of wasteful.
• The HttpException is not traced as-is as there is no instance for Eq HttpException, and Trace requires it. A String is used instead. Not ideal, but might be enough for tests? Removing Eq for Trace would be a breaking change.
• New dependencies that may not be useful for everyone is introduced.

[michivi]

Sorry, the code was only working if the test program is not in a container. Let me just fix this so it works both in a container, and on the host.

[michivi]

Ok, the waitForHttp function now works within and outside of a Docker network:

• If the test program is running inside the Docker network, assume we can call it directly using its alias. It's not applicable though if the test program is not in the same network as the container. That will be the developer's responsibility. We cannot use the host address as the exposed port is not public.
• If the test program is running directly on the host, then we use the exposed port as before.