[MERGE] repository external reference tests (StackedBranches)

[Ian Clatworthy]

Number 2 in the series of patches for stacked branches.
Robert put this up for review earlier and it was reviewed
by Andrew and Martin. I believe some of the requested tweaks
still need to be done (though I'm guessing Andrew could
do those much faster/safer than I given his knowledge of
exactly where the APIs in question are up to in their
evolution). I have made some changes though to Robert's
code to merge in vila's preferred new way of loading tests
faster.

Given the above, I'm not actually going to review this code
myself. Could Andrew or Martin re-review this please?

Ian C.
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