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: http://postgr.es/m/OS0PR01MB571626984BD099DADF53F38394899@OS0PR01MB5716.jpnprd01.prod.outlook.com