Improve stability of identity.sql regression test.

I noticed while trying to run the regression tests under a low
geqo_threshold that one query on information_schema.columns had
unstable (as in, variable from one run to the next) output order.
This is pretty unsurprising given the complexity of the underlying
plan.  Interestingly, of this test's three nigh-identical queries on
information_schema.columns, the other two already had ORDER BY clauses
guaranteeing stable output.  Let's make this one look the same.

Back-patch to v10 where this test was added.  We've not heard field
reports of the test failing, but this experience shows that it can
happen when testing under even slightly unusual conditions.

src/test/regress/expected/identity.out

@@ -335,7 +335,7 @@ SELECT * FROM itest6;
  102 | 
 (3 rows)
 
-SELECT table_name, column_name, is_identity, identity_generation FROM information_schema.columns WHERE table_name = 'itest6';
+SELECT table_name, column_name, is_identity, identity_generation FROM information_schema.columns WHERE table_name = 'itest6' ORDER BY 1, 2;
  table_name | column_name | is_identity | identity_generation 
 ------------+-------------+-------------+---------------------
  itest6     | a           | YES         | BY DEFAULT

src/test/regress/sql/identity.sql

@@ -208,7 +208,7 @@ INSERT INTO itest6 DEFAULT VALUES;
 INSERT INTO itest6 DEFAULT VALUES;
 SELECT * FROM itest6;
 
-SELECT table_name, column_name, is_identity, identity_generation FROM information_schema.columns WHERE table_name = 'itest6';
+SELECT table_name, column_name, is_identity, identity_generation FROM information_schema.columns WHERE table_name = 'itest6' ORDER BY 1, 2;
 
 ALTER TABLE itest6 ALTER COLUMN b SET INCREMENT BY 2;  -- fail, not identity