Test with a kernel that doesn't support Landlock

[l0kod]

Ubuntu 18 is useful because it provides a kernel that doesn't support Landlock. Because of actions/runner-images#6002, we need to replace it with a more generic approach, probably with a dedicated kernel, which is not trivial because this implies to manage a full VM.

[l0kod]

This leads to this issue: #22 https://github.com/landlock-lsm/rust-landlock/actions/runs/3846419818/jobs/6551698849

[l0kod]

#39 removed the dependency, but we still need to test with LANDLOCK_CRATE_TEST_ABI: 0.

Thanks to torvalds/linux@74ce793, it is now possible to fully use UML to test Landlock. I have code that I need to clean up a bit before creating a PR.

[l0kod]

Handled with #41