Write an e2e test for upstream client-cert authn

[stlaz]

#211 introduced client-cert authentication of the proxy towards upstream.

#211 (review) requests an e2e test. The PR already introduces a unit-test that checks that the upstream transport is set up correctly. It would still be nice to have an e2e test to check that client-cert auth works well towards upstream.

This task includes:

1. finding the right container image to test the client-cert auth with
2. configuring the image to force-require client cert auth