mkosi-initrd: Port tests from old repository

mkosi-initrd: Port tests from old repository

This commit ports some of the tests from
https://github.com/systemd/mkosi-initrd/blob/main/.github/workflows/build-fedora.sh
over.

The LUKS test is modified to generate the LUKS root partition using
repart instead of doing it manually. For the LVM tests we're forced
to do it manually as systemd-repart doesn't support setting up LVM
(and probably never will).

We also add an initrd size test so we notice when initrds grow due
to distribution packaging changes.