Add an integration test or two

[da1910]

For one of our projects we use this library in a container setup that allows us to test client authentication against a "real" kerberized host. It would be relatively trivial to adapt that setup to test this library against kerberos clients.

See https://github.com/pyansys/openapi-common for our approach for client testing. I can make the dockerfiles for the KDC available if you like.