This seems to cause unpredictable failures with pastebin (at least).
Tests will have the backends attribute if they really want to tests all
backends at once. For now, one backend is chosen for each run (and since
tests are ran often, it will cover all backends eventually.) The backend
instance name is displayed to ease the debugging.
This is useful for testing backends with different configurations
(different sites, being anonymous or not, etc.)
The previous behavior was to chose a random backend instance.
