test: test folder renames

In [1] Mark Walters reported a problem with messages being removed
from the database when the parent directory was renamed. Jani Nikula
proposed [2] these tests but observed

   This test is not suitable for merging since it's not deterministic.

After applying Jani's patch [3], the tests now pass deterministically,
and could usefully act as regression tests.

[1]: id:87siray6th.fsf@qmul.ac.uk
[2]: id:1393191650-28333-1-git-send-email-jani@nikula.org
[3]: id:1441445731-4362-2-git-send-email-jani@nikula.org