Improve DDL deparsing test

Improve DDL deparsing test

1. The test initially focuses on the "parent" table, then switches to
the "part" table, and goes back to the "parent" table. That seems a
little weird, so move the tests around so that all the commands on the
"parent" table are done first, followed by the "part" table.

2. ALTER TABLE ALTER COLUMN SET EXPRESSION was not tested, so add
that.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/CACJufxFDi7fnwB-8xXd_ExML7-7pKbTaK4j46AJ=4-14DXvtVg@mail.gmail.com