stat: Nits in readlink tests

• The f_flag test may fail if a component of the full path to the temporary directory is a symbolic link.

• The n_flag test had an empty head; give it a description.

• Use consistent quoting.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D56293