Added unit test for Library unicode filenames #833

This is a dupe of https://github.com/sampsyo/beets/issues/757
which was fixed in https://github.com/sampsyo/beets/commit/180433e72c8a07d98573363020a74f07d66c02ed

I ended up coming up with the fix independently and along the way created a unit test...
And this one.