Fix alter_table.sql test case to test what it claims to.

The stanza "SET STORAGE may need to add a TOAST table" does not
test what it's supposed to, and hasn't done so since we added
the ability to store constant column default values as metadata.
We need to use a non-constant default to get the expected table
rewrite to actually happen.

Fix that, and add the missing checks that would have exposed the
problem to begin with.

Noted while reviewing a patch that made changes in this test case.
Back-patch to v11 where the problem came in.

src/test/regress/expected/alter_table.out

@@ -2245,12 +2245,26 @@ alter table recur1 alter column f2 type recur2; -- fails
 ERROR:  composite type recur1 cannot be made a member of itself
 -- SET STORAGE may need to add a TOAST table
 create table test_storage (a text, c text storage plain);
+select reltoastrelid <> 0 as has_toast_table
+  from pg_class where oid = 'test_storage'::regclass;
+ has_toast_table 
+-----------------
+ t
+(1 row)
+
 alter table test_storage alter a set storage plain;
-alter table test_storage add b int default 0; -- rewrite table to remove its TOAST table
+-- rewrite table to remove its TOAST table; need a non-constant column default
+alter table test_storage add b int default random()::int;
+select reltoastrelid <> 0 as has_toast_table
+  from pg_class where oid = 'test_storage'::regclass;
+ has_toast_table 
+-----------------
+ f
+(1 row)
+
 alter table test_storage alter a set storage extended; -- re-add TOAST table
 select reltoastrelid <> 0 as has_toast_table
-from pg_class
-where oid = 'test_storage'::regclass;
+  from pg_class where oid = 'test_storage'::regclass;
  has_toast_table 
 -----------------
  t
@@ -2265,10 +2279,10 @@ alter table test_storage alter column a set storage external;
 \d+ test_storage
                                      Table "public.test_storage"
  Column |  Type   | Collation | Nullable |      Default      | Storage  | Stats target | Description 
---------+---------+-----------+----------+---------+----------+--------------+-------------
+--------+---------+-----------+----------+-------------------+----------+--------------+-------------
  a      | text    |           |          |                   | external |              | 
  c      | text    |           |          |                   | plain    |              | 
- b      | integer |           |          | 0       | plain    |              | 
+ b      | integer |           |          | random()::integer | plain    |              | 
 Indexes:
     "test_storage_idx" btree (b, a)
 

src/test/regress/sql/alter_table.sql

@@ -1528,13 +1528,16 @@ alter table recur1 alter column f2 type recur2; -- fails
 
 -- SET STORAGE may need to add a TOAST table
 create table test_storage (a text, c text storage plain);
-alter table test_storage alter a set storage plain;
-alter table test_storage add b int default 0; -- rewrite table to remove its TOAST table
-alter table test_storage alter a set storage extended; -- re-add TOAST table
-
 select reltoastrelid <> 0 as has_toast_table
-from pg_class
-where oid = 'test_storage'::regclass;
+  from pg_class where oid = 'test_storage'::regclass;
+alter table test_storage alter a set storage plain;
+-- rewrite table to remove its TOAST table; need a non-constant column default
+alter table test_storage add b int default random()::int;
+select reltoastrelid <> 0 as has_toast_table
+  from pg_class where oid = 'test_storage'::regclass;
+alter table test_storage alter a set storage extended; -- re-add TOAST table
+select reltoastrelid <> 0 as has_toast_table
+  from pg_class where oid = 'test_storage'::regclass;
 
 -- check STORAGE correctness
 create table test_storage_failed (a text, b int storage extended);