We were using stretchr/testify and test-go/testify inconsitently and
sometimes mixed in the same tests. This can cause very strange issue,
e.g when using things like mock.MatchedBy().
This moves all our code to stretchr/testify, which seems to be far
more active and maintained then test-go/testify.
The old approach of the authentication middlewares had the problem that when an authenticator could not authenticate a request it would still send it to the next handler, in case that the next one can authenticate it. But if no authenticator could successfully authenticate the request, it would still be handled, which leads to unauthorized access.
