add CI coverage for `kola testiso --pxe-append-rootfs`

[dustymabe]

Currently we don't have any CI coverage for the PXE case:

Concatenate the initramfs and rootfs files together, and specify the combined file as the initrd. This method is slower and requires 4 GiB of RAM.

from the docs.

The kola testiso command supports --pxe-append-rootfs (underlying code), but we don't set that option when we run any of our default tests. We either need to create a new name for this type of test and add it to the default list OR we need to just blanket add it to a class of tests (like the PXE offline ones). I'm not sure which would be best.