tests: Work around Btrfs handling of nlink for empty directories.

Fixes <http://bugs.gnu.org/21280>.
Reported by Leo Famulari <leo@famulari.name>.

* tests/nar.scm (file-tree-equal?): Use #t as the initial result.  Btrfs would
  set nlink to 1 instead of 2 for an empty directory, thereby leading
  'file-tree-equal?' to return #f.