pgsql: Add regression tests for INSERT/UPDATE+RETURNING

Add regression tests for INSERT/UPDATE+RETURNING

This adds regressions tests which are specific to INSERT+RETURNING and
UPDATE+RETURNING to ensure that the SELECT policies are added as
WithCheckOptions (and should therefore throw an error when the policy is
violated).

Per suggestion from Andres.

Back-patch to 9.5 as the prior commit was.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/be400cd25c7f407111b9617dbf6a5fae761754cb

Modified Files
--------------
src/test/regress/expected/rowsecurity.out |   69 +++++++++++++++++++++++++++++
src/test/regress/sql/rowsecurity.sql      |   63 ++++++++++++++++++++++++++
2 files changed, 132 insertions(+)