Rework mantle kola tests for Torcx removal

[pothos]

In flatcar/scripts#982 flatcar/scripts#1216 the use of a systemd-sysext image placed on the /usr partition is prepared together with the removal of Torcx. To get this merged we also need to remove Torcx specifics from the ci-automation and rework the mantle kola tests.
Torcx tests would only be run for old Flatcar versions. Docker tests with Torcx-specifics should be ported to work with the systemd-sysext image. New tests should be added to check that disabling the Docker sysext image works (through the documented /dev/null symlink).