Move privilege check for SET SESSION AUTHORIZATION.

Move privilege check for SET SESSION AUTHORIZATION.

Presently, the privilege check for SET SESSION AUTHORIZATION is
performed in session_authorization's assign_hook.  A relevant
comment states, "It's OK because the check does not require catalog
access and can't fail during an end-of-transaction GUC
reversion..."  However, we plan to add a catalog lookup to this
privilege check in a follow-up commit.

This commit moves this privilege check to the check_hook for
session_authorization.  Like check_role(), we do not throw a hard
error for insufficient privileges when the source is PGC_S_TEST.

Author: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com