Add new RLS tests to test policies applied by command type.

Add new RLS tests to test policies applied by command type.

The existing RLS tests focus on the outcomes of various testing
scenarios, rather than the exact policies applied. This sometimes
makes it hard to see why a particular result occurred (e.g., which
policy failed), or to construct a test that fails a particular policy
check without an earlier check failing. These new tests issue NOTICE
messages to show the actual policies applied for each command type,
including the different paths through INSERT ... ON CONFLICT and
MERGE, making it easier to verify the expected behaviour.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Viktor Holmberg <v@viktorh.net>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWqnfeChjK=n1V_dYZT4rt4mnq+ybf9c0qXDYTVMsy8pg@mail.gmail.com