We support two types of passing credentials to the proxy, either via the
URL or explicitly by specifying user and password. We test these types
by modifying the proxy URL and executing the tests twice, which is
in fact unnecessary and requires us to maintain the list of environment
variables and test executions across multiple CI infrastructures.

To fix the situation, we can just always pass the host, port, user and
password to the tests. The tests can then assemble the complete URL
either with or without included credentials, allowing us to test both
cases in-process.