tons of copy-pasting going on in here...

better add a utility method to call from the various `test` methods?

```python
def extract_something(self, archive_file):
    out = ex.extract_archive(archive_file, self.tmpdir)
    out_file = os.path.join(out, 'test.txt')
    self.assertEqual(open(out_file).read(), 'test ok\n')
```