Fix the error hint message and test for reset_shared with unknown stats type

<div><div><div>Hi hackers,</div><div> </div><div>The errhint() in pg_stat_reset_shared() were missing the fact that recently added in [1] new lock stats kind.</div><div>The patch fixes error hint message and regress test for reset_shared with unknown stats type.</div><div> </div><div>[1]: https://www.postgresql.org/message-id/aIyNxBWFCybgBZBS%40ip-10-97-1-34.eu-west-3.compute.internal</div><div> </div><div>Best regards,</div><div>Maksim Logvinenko</div><div>Postgres Professional: http://postgrespro.com</div></div></div>