[MERGE][1.2?][Bug #137823] Don't blindly raise UnavailableFeature during test setup

[Martin Pool]

Martin Pool has voted approve.
Status is now: Resubmit
Comment:
The fix looks correct and I think this is reasonable for 1.2.

Robert, Vila did have a note about the change of logging.

I wonder if we should specially distinguish tests that failed to setup, 
or even stop immediately if it happens.  setup/teardown is such an awful 
api anyhow.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Cm2zlu8x0b9.fsf%40free.fr%3E