Add alter_generic regression test

This makes refactoring of parts of the ALTER command safe(r) because we
ensure no change in functionality.

Author: KaiGai Kohei

src/test/regress/parallel_schedule

@@ -101,7 +101,7 @@ test: select_views portals_p2 foreign_key cluster dependency guc bitmapops combo
 # NB: temp.sql does a reconnect which transiently uses 2 connections,
 # so keep this parallel group to at most 19 tests
 # ----------
-test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table sequence polymorphism rowtypes returning largeobject with xml
+test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table alter_generic sequence polymorphism rowtypes returning largeobject with xml
 
 # run stats by itself because its delay may be insufficient under heavy load
 test: stats

src/test/regress/serial_schedule

@@ -125,6 +125,7 @@ test: without_oid
 test: conversion
 test: truncate
 test: alter_table
+test: alter_generic
 test: sequence
 test: polymorphism
 test: rowtypes