tests: Exercise host-os, host-etc and os-release in fake system roots
tests: Exercise host-os, host-etc and os-release in fake system roots

Now that FlatpakExports can be told to look for /etc and /usr in a
fake directory hierarchy, we can assert that systems resembling
particular OSs' layouts still work, even if we are not running on that
OS right now. In particular, this provides unit tests for commits
12e3dc05, 08d65c54, 7872935e and fe2536b8.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak