I put these in `conftest`, because the py.test docs specifically recommend that as a place to put fixtures that are shared between test modules [1], but as I don't think we *must* do it that way.

`conftest` is a special name for `py.test` which is something that always gets imported and run, but does not contain tests.  We could explicitly import it, just to be more clear, but that wouldn't be necessary.  Or we can rename it to something else (as long as "test" isn't in the name), and then we'd be required to import it (which might be clearer in the end).

[1] http://pytest.org/latest/fixture.html#sharing-a-fixture-across-tests-in-a-module-or-class-session