Fix tests on vagrant shared folder.

Fixes #111.

It would be good to also have instructions on how to run the tests so that the temporary directory isn't a vagrant shared folder, but I'm not sure how to tell tox to put the temporary directory somewhere other than `.tox`.
I certainly have problems with symlinks in VirtualBox shared folders. It reports EROFS when trial attempts to create a symlink for its lockfile. See https://www.virtualbox.org/ticket/10085#comment:14