Still WIP, can't get the validation to work. Given that both ostree and reboot tests have special detection, that's... expected, I guess

Here's a couple of findings related to earlier discussions with @VladimirSlavik which should likely be taken into account when finishing the test:

• The container image used in the test doesn't likely support BTRFS by default (no dracut module in initrd), so it will likely fail to mount root fs.
• /boot partition may need to have a boot label set, so that kernel and initrd files are properly found by GRUB.
• not sure if GRUB will find kernel and initrd files if /boot doesn't reside on a separate partition

Taking over the test development from Vladimir, will continue working on it after some higher priority tasks I've on my plate right now are done.

Thank you Jirko, I think eventually this test will replace the rpm-ostree-container-rhel9 which does not test booting of the target system, just presence of the payload). I am fixing it for now in #1052.

FYI, there is some tests renaming / consolidation undergoing: #1054

Follow-up in #1096