Prevent fails of tests when path contains dots
Prevent fails of tests when path contains dots

Unit test may fails when path to temprorary library contains `.`; to
garantue that bug wasn't here, it forces to use one more `.` inside path.

Fixes: https://github.com/beetbox/beets/issues/4151

beetbox/beets,beetbox/beets,beetbox/beets,beetbox/beets