Rename *_implementations tests to per_*

   >> >>>
   >> >>> I'm going to rename the module to per_repository_reference which is
   >> >>> consistent with at least per_lock and much more wieldy.
   >> >>
   >> >> Its inconsistent with all the *_implementations ones though. Can we pick
   >> >> one standard and use it? I don't care which.
   >>
   martin> My vote is for "per_*" then; it's shorter and imo conveys the most
   martin> important information which is that they're multiplied tests.
   >>
   >> +1 for the same resaons
   >>
   >> Vincent
   >>
   >>

   john> Agreed.

   john> Of course, now we need to decide if it is worth
   john> renaming everything just for this? We certainly don't
   john> have a history of large "cleanup" changes. (All of the
   john> "from ... import" lines need to change, etc.)

   john> As it is is the test suite, I think it is okay, though.

Whatever we decide, I think we should at least file a bug for it
and maybe add some FIXME here and there, so that a newcomer will
have a hint or two about what is the way we want things to be
done and don't get puzzled by finding several different ways in
the code base.