Fix (some) tests

Still working on it, but thought I'd make a PR so I can ping against Travis in addition to local testing.

Currently at: http://sprunge.us/bBbi

It looks like it's not properly sandboxing some tests, e.g., for `AssertionError: u'/home/freso/.config/beets/beets.db' != '/tmp/tmpIIRxfP/.config/beets/beets.db'` and the fact that it's trying to load some modules listed in ~/.config/beets/config.yaml.