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.

tests/nar.scm

@@ -121,7 +121,7 @@
                     (lambda (name stat errno result)
                       (pk 'error name stat errno)
                       #f)
-                    (> (stat:nlink (stat output)) 2)
+                    #t                            ; result
                     input
                     lstat))