Re-enable http client unit tests, with local web-service

[murerfel]

In PR #949, commit 9bd7cf8 I disabled unit tests that depend on various external web-services to work. That step was necessary because CI failed too often because those services were unavailable.

However, these tests are valuable and we should find a way to run them on local resources. One idea e.g. for the httpbin service, is to run its docker image locally:

docker run -p 80:80 kennethreitz/httpbin

This could be achieved by using a crate like testcontainers that allows running docker containers from inside tests. So the idea is to run a httpbin service container instance for each of those tests (remember, cargo test runs parallel, so make sure they don't interfere) and then run the test http client against that local container instance.