add bootloader update check to the upgrade test

[dustymabe]

As part of:

• https://fedoraproject.org/wiki/Changes/AutomaticBootloaderUpdatesBootc
• Enable automatic bootloader updates fedora-coreos-tracker#1468

The bootloader is now getting updated on every boot (in f43 +)

Let's augment the upgrade test to

1. verify the bootloader is getting updated
2. perform an extra reboot to actually test booting with the updated bootloader

For now we'd have to conditionalize this on F43 somehow, since the change has only taken effect there.