Sync some improvements from cockpit-machines:
* Unbreak running `tmt` in local trees with an already existing ./bots
symlink.
* Don't install all NPM packages, only chrome-remote-interface and sizzle.
* Disable detection of affected tests, taking too long for real
projects.
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.