Add test for MAINTAIN permission with pg_restore_extended_stats()

Add test for MAINTAIN permission with pg_restore_extended_stats()

Like its cousin functions for the restore of relation and attribute
stats, pg_restore_extended_stats() needs to be run by a user that is the
database owner or has MAINTAIN privileges on the table whose stats are
restored.  This commit adds a regression test ensuring that MAINTAIN is
required when calling the function.  This test also checks that a
ShareUpdateExclusive lock is taken on the table whose stats are
restored.

This has been split from the commit that has introduced
pg_restore_extended_stats(), for clarity.

Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com