Add negative test case for creating an index on external table.

You can't do it, but it would still be good to have a test case for it.
Spotted while reviewing legacy "cdbfast" tests - there was a test for this
there.

src/test/regress/input/external_table.source

@@ -127,6 +127,7 @@ truncate ext;
 delete from ext;
 update ext set x='1' where x='2';
 insert into ext(x) values('a');
+create index ext_index on ext(x); -- should fail
 drop table ext; -- should fail (wrong object)
 drop external table ext;
 

src/test/regress/output/external_table.source

@@ -207,6 +207,8 @@ update ext set x='1' where x='2';
 ERROR:  cannot update or delete from external relation "ext"
 insert into ext(x) values('a');
 ERROR:  cannot change a readable external table "ext"
+create index ext_index on ext(x); -- should fail
+ERROR:  cannot create indexes on external tables.
 drop table ext; -- should fail (wrong object)
 ERROR:  "ext" is not a base table
 HINT:  Use DROP EXTERNAL TABLE to remove an external table