Add kola tests for unique bootfs work

[jlebon]

We've added new services in https://github.com/coreos/fedora-coreos-config/pull/1269 and https://github.com/coreos/fedora-coreos-config/pull/1540 which verify that there is only one boot filesystem. We currently don't have CI that exercises them.

Add tests for these. I see at least one for each:

1. we attach a block device which has a pre-existing boot filesystem on it, and verify that coreos-unique-boot.service fails as expected.
2. we boot with an Ignition config which creates a second boot filesystem and verify that coreos-ignition-unique-boot.service fails as expected.

We have at least one test already which verifies Ignition failure in the initrd (see qemufailure.go). Using that code as a base for the new tests would be a good starting point.