Teardown files for test_dir

[gaborcsardi]

As per #157. If you think this can make it, then I'll add a test case as well.

[hadley]

Let's discuss this a bit more - the granularity here seems wrong, because I think you'd want to run the teardown after each file, not after everything.

[gaborcsardi]

As for the granularity, I think it is right. I understand that you may want a teardown for each file, but you can already do that, see my example at #157, and as it turns out on.exit works as well.

On the other hand, if you want a teardown for all tests in a directory, then there is really no good way to do that. I think this also has legitimate use cases, see e.g. the DB example in #157.

Furthermore, this way teardown functions are symmetric to helper functions. I think it would be unintuitive to have helper run for the directory and teardown for each file. This is if we think of helpers and teardowns as processes operating on the same level.

[hadley]

Helper really doesn't seem symmetric to tear down to me. Let's keep thinking about this - I need to get a new release out v soon to fix some r devel problems, but I think this is too big to go in this version.