Add link count test for root inode

Add tests for
97aa3ba44("Fix link count of root inode when snapdir is visible")
as suggested in #8727.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8732