Add tuple deformation test for virtual generated columns

Add coverage for a virtual generated NOT NULL column followed by a
physically stored NOT NULL column.  This exercises the tuple deformation
case fixed by 89eafad297, where TupleDescFinalize() could incorrectly
treat a virtual generated column as part of the guaranteed physical column
prefix and compute cached offsets past it.

Without that fix, deforming the following column could read from the wrong
tuple offset.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/A4BC563C-0CA3-4EF3-952A-EA41F9E5BF1E%40gmail.com

src/test/regress/expected/generated_stored.out

@@ -721,6 +721,11 @@ ERROR:  null value in column "b" of relation "gtest21b" violates not-null constr
 DETAIL:  Failing row contains (null, null).
 ALTER TABLE gtest21b ALTER COLUMN b DROP NOT NULL;
 INSERT INTO gtest21b (a) VALUES (0);  -- ok now
+-- virtual generated columns are not physically stored, even when not null
+--CREATE TABLE gtest21c (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) VIRTUAL NOT NULL, c int NOT NULL);
+--INSERT INTO gtest21c (a, c) VALUES (10, 42);
+--SELECT a, b, c FROM gtest21c;
+--DROP TABLE gtest21c;
 -- not-null constraint with partitioned table
 CREATE TABLE gtestnn_parent (
     f1 int,

src/test/regress/expected/generated_virtual.out

@@ -727,6 +727,16 @@ ERROR:  null value in column "b" of relation "gtest21b" violates not-null constr
 DETAIL:  Failing row contains (null, virtual).
 ALTER TABLE gtest21b ALTER COLUMN b DROP NOT NULL;
 INSERT INTO gtest21b (a) VALUES (0);  -- ok now
+-- virtual generated columns are not physically stored, even when not null
+CREATE TABLE gtest21c (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) VIRTUAL NOT NULL, c int NOT NULL);
+INSERT INTO gtest21c (a, c) VALUES (10, 42);
+SELECT a, b, c FROM gtest21c;
+ a  | b  | c  
+----+----+----
+ 10 | 20 | 42
+(1 row)
+
+DROP TABLE gtest21c;
 -- not-null constraint with partitioned table
 CREATE TABLE gtestnn_parent (
     f1 int,

src/test/regress/sql/generated_stored.sql

@@ -368,6 +368,11 @@ INSERT INTO gtest21b (a) VALUES (NULL);  -- error
 ALTER TABLE gtest21b ALTER COLUMN b DROP NOT NULL;
 INSERT INTO gtest21b (a) VALUES (0);  -- ok now
 
+-- virtual generated columns are not physically stored, even when not null
+--CREATE TABLE gtest21c (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) VIRTUAL NOT NULL, c int NOT NULL);
+--INSERT INTO gtest21c (a, c) VALUES (10, 42);
+--SELECT a, b, c FROM gtest21c;
+--DROP TABLE gtest21c;
 -- not-null constraint with partitioned table
 CREATE TABLE gtestnn_parent (
     f1 int,

src/test/regress/sql/generated_virtual.sql

@@ -374,6 +374,12 @@ INSERT INTO gtest21b (a) VALUES (NULL);  -- error
 ALTER TABLE gtest21b ALTER COLUMN b DROP NOT NULL;
 INSERT INTO gtest21b (a) VALUES (0);  -- ok now
 
+-- virtual generated columns are not physically stored, even when not null
+CREATE TABLE gtest21c (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) VIRTUAL NOT NULL, c int NOT NULL);
+INSERT INTO gtest21c (a, c) VALUES (10, 42);
+SELECT a, b, c FROM gtest21c;
+DROP TABLE gtest21c;
+
 -- not-null constraint with partitioned table
 CREATE TABLE gtestnn_parent (
     f1 int,