This simplifies the "outside" setup quite a bit and gives us the same
tasks container that test runs on the Cockpit CI run under.
This change also implicitly changes the test browser to chromium (which
is no longer an issue — we just use it from the tasks container).
The tests switch to German, make sure that the corresponding locale
exists. Otherwise, tests will run into unexpected messages like "invalid
or unusable locale: de_DE.UTF-8".
Copy and adjust the browser.sh and run-test.sh scripts from
cockpit-podman, which we have used in Fedora/RHEL dist-git gating and
packit tests for a while.
