Disallow changing NO INHERIT status of a not-null constraint

Disallow changing NO INHERIT status of a not-null constraint

It makes no sense to add a NO INHERIT not-null constraint to a child
table that already has one in that column inherited from its parent.
Disallow that, and add tests for the relevant cases.

Per complaint from Kyotaro Horiguchi.  I also used part of his proposed
patch.

Co-authored-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20230828.161658.1184657435220765047.horikyota.ntt@gmail.com