Add regression tests for REPLICA IDENTITY with dropped indexes

REPLICA IDENTITY USING INDEX behaves the same way as NOTHING if the
associated index is dropped, even if there is a primary key that could
be used as a fallback for the changes generated.  There have never been
any tests to cover such scenarios, so this commit closes the gap.

Author: Michael Paquier
Reviewed-by: Masahiko Sawada, Rahila Syed, Euler Taveira
Discussion: https://postgr.es/m/20200522035028.GO2355@paquier.xyz

contrib/test_decoding/sql/ddl.sql

@@ -345,6 +345,37 @@ UPDATE table_with_unique_not_null SET id = -id;
 UPDATE table_with_unique_not_null SET id = -id;
 DELETE FROM table_with_unique_not_null WHERE data = 3;
 
+-- check tables with dropped indexes used in REPLICA IDENTITY
+-- table with primary key
+CREATE TABLE table_dropped_index_with_pk (a int PRIMARY KEY, b int, c int);
+CREATE UNIQUE INDEX table_dropped_index_with_pk_idx
+  ON table_dropped_index_with_pk(a);
+ALTER TABLE table_dropped_index_with_pk REPLICA IDENTITY
+  USING INDEX table_dropped_index_with_pk_idx;
+DROP INDEX table_dropped_index_with_pk_idx;
+INSERT INTO table_dropped_index_with_pk VALUES (1,1,1), (2,2,2), (3,3,3);
+UPDATE table_dropped_index_with_pk SET a = 4 WHERE a = 1;
+UPDATE table_dropped_index_with_pk SET b = 5 WHERE a = 2;
+UPDATE table_dropped_index_with_pk SET b = 6, c = 7 WHERE a = 3;
+DELETE FROM table_dropped_index_with_pk WHERE b = 1;
+DELETE FROM table_dropped_index_with_pk WHERE a = 3;
+DROP TABLE table_dropped_index_with_pk;
+
+-- table without primary key
+CREATE TABLE table_dropped_index_no_pk (a int NOT NULL, b int, c int);
+CREATE UNIQUE INDEX table_dropped_index_no_pk_idx
+  ON table_dropped_index_no_pk(a);
+ALTER TABLE table_dropped_index_no_pk REPLICA IDENTITY
+  USING INDEX table_dropped_index_no_pk_idx;
+DROP INDEX table_dropped_index_no_pk_idx;
+INSERT INTO table_dropped_index_no_pk VALUES (1,1,1), (2,2,2), (3,3,3);
+UPDATE table_dropped_index_no_pk SET a = 4 WHERE a = 1;
+UPDATE table_dropped_index_no_pk SET b = 5 WHERE a = 2;
+UPDATE table_dropped_index_no_pk SET b = 6, c = 7 WHERE a = 3;
+DELETE FROM table_dropped_index_no_pk WHERE b = 1;
+DELETE FROM table_dropped_index_no_pk WHERE a = 3;
+DROP TABLE table_dropped_index_no_pk;
+
 -- check toast support
 BEGIN;
 CREATE SEQUENCE toasttable_rand_seq START 79 INCREMENT 1499; -- portable "random"