pgsql: Expand tests of test_ddl_deparse/ for ALTER TABLE

Expand tests of test_ddl_deparse/ for ALTER TABLE

This module is expanded to track the description of the objects changed
in the subcommands of ALTER TABLE by reworking the function
get_altertable_subcmdtypes() (now named get_altertable_subcmdinfo) used
in the event trigger of the test.  It now returns a set of rows made of
(subcommand type, object description) instead of a text array with only
the information about the subcommand type.

The tests have been lacking a lot of the subcommands added to
AlterTableType over the years.  All the missing subcommands are added,
and the code is now structured so as the addition of a new subcommand
is detected by removing the default clause used in the switch for the
subcommand types.

The coverage of the module is increased from roughly 30% to 50%.  More
could be done but this is already a nice improvement.

Author: Michael Paquier, Hou Zhijie
Reviewed-by: Álvaro Herrera, Amit Kapila, Hayato Kuroda
Discussion: https://postgr.es/m/OS0PR01MB571626984BD099DADF53F38394899@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/07ff701dbd53aeb86dd5a09a8b943b3782d4b67f

Modified Files
--------------
.../test_ddl_deparse/expected/alter_table.out      | 126 ++++++++++++++++++++-
.../test_ddl_deparse/expected/create_table.out     |   8 +-
.../test_ddl_deparse/expected/create_view.out      |   2 +-
.../test_ddl_deparse/expected/test_ddl_deparse.out |   4 +-
.../modules/test_ddl_deparse/sql/alter_table.sql   |  56 +++++++++
.../test_ddl_deparse/sql/test_ddl_deparse.sql      |   4 +-
.../test_ddl_deparse/test_ddl_deparse--1.0.sql     |   6 +-
.../modules/test_ddl_deparse/test_ddl_deparse.c    |  73 +++++++++---
8 files changed, 250 insertions(+), 29 deletions(-)