Cover PGPRO-1401 by regression test.

Author: Alexander Korotkov

contrib/in_memory/expected/subtrans.out

@@ -290,3 +290,10 @@ SELECT min(id), count(id), val FROM im_test1 GROUP BY val ORDER BY min(id);
    2 |   250 | valxxxyyyz
 (2 rows)
 
+TRUNCATE im_test1;
+BEGIN;
+SAVEPOINT s1;
+SET TRANSACTION READ ONLY;
+SET TRANSACTION READ WRITE;
+ERROR:  cannot set transaction read-write mode inside a read-only transaction
+ROLLBACK;

contrib/in_memory/sql/subtrans.sql

@@ -145,3 +145,10 @@ RELEASE s1;
 SELECT min(id), count(id), val FROM im_test1 GROUP BY val ORDER BY min(id);
 COMMIT;
 SELECT min(id), count(id), val FROM im_test1 GROUP BY val ORDER BY min(id);
+
+TRUNCATE im_test1;
+BEGIN;
+SAVEPOINT s1;
+SET TRANSACTION READ ONLY;
+SET TRANSACTION READ WRITE;
+ROLLBACK;