[MERGE] Extract the enumeration of the branch scenarios from the load_tests function in

[Michael Hudson]

Robert Collins wrote:
> bb:reject
> 
> I landed this from your patch before.

Thanks.. but as mentioned on IRC, is there a chance this could be cherry
picked onto the 1.13 branch too?  That would mean that we could use 1.13
final when that rolls around for Launchpad.

Cheers,
mwh