The current hooks do not have a good testing story. To fix this
this PR adds support for "hook/foo.test" files that will be run
at the end of the prime phase during the snap build.

It also adds two initial tests and converts the existing ABI test.
If this looks reasonable we can write tests for the remaining hooks
to ensure we do not regress again.

Only run hook tests when root otherwise the chroot to the prime
dir will not work.