Add tests to verify we support security invoker views

[naisila]

PG15 added support for security invoker views. Relevant PG conmit: https://github.com/postgres/postgres/commit/7faa5fc84bf46ea6c543993cffb8be64dff60d25

These views check the permissions for the underlying tables of the view invoker user, not the view definer user.

When the view has underlying distributed tables, the queries to the shards are sent by opening connections with the current user, which is the view invoker, no matter what the type of the view is. This means that, for distributed views, they were always behaving like security invoker views. Check the following issue for more details: https://github.com/citusdata/citus/issues/6161 So, Citus doesn't fully support security definer views.

However Citus does fully support security invoker views. We add tests to make sure we cover different cases.