test_pack_repository does not test 2a

[Andrew Bennetts]

test_pack_repository has a scenario list that doesn't include 2a. Adding 2a causes 3 test breakages (but shallow ones, I suspect.)

Beyond simply adding 2a to the scenario list and making the tests pass, there's a couple of issues here:
 - test_pack_repository should be named per_* so that it isn't missed when a dev looks for places to update test parameterisation for a new format (or audit existing coverage).
 - the scenario list weirdly duplicates a bunch of information that could be much more simply plucked from format objects. (i.e. why not define the scenarios by bzrdir_format registry names only, and automatically derive the attributes from there?)
 - perhaps there should not be a manual scenario list at all, ideally we should generate it from the complete list of formats, like per_repository_reference does.

i.e. it would be good to not just fix the immediate bug, but try to fix what allowed it to happen in the first place.