[merge] clean up/separate pack repository tests

[Martin Pool]

* move pack-repository specific tests from test_repository into their own file
* they were using multiple inheritance to generate the different
scenarios that need to be tested; now they use test scenarios, which I
think makes it clearer what is happening
* I also run them for the 1.6 formats

There are no substantive changes to the tests themselves.

I could have put this in repository_implementations and skipped it for
non-pack repositories, but that seems ugly.  I would rather take it
the other way in fact and split the ones that only run for stacking
pack repositories into a file just concerned with that aspect, and I
might do that later if this is accepted.

-- 
Martin <http://launchpad.net/~mbp/>