cleanups not called when setUp fails when a fixture is used as a context

[Cedric Brandily]

When we do:

  with fixture2:
    pass

fixture2.__exit__ is called only if fixture2.__enter__ succeed, it implies fixture2 cleanups (done in __exit__[1]) are only done if fixture2 setUp succeed (done in __enter__[1])

[1] http://bazaar.launchpad.net/~testing-cabal/python-fixtures/master/view/head:/fixtures/fixture.py

[Robert Collins (lifeless)]

I wanted to capture the logic here for easy later review..., so I did a blog post. https://rbtcollins.wordpress.com/2015/06/22/revisiting-the-fixture-api-handling-leaky-resources/