Add regression test for TRUNCATE on foreign table not supporting TRUNCATE.

file_fdw doesn't support INSERT, UPDATE, DELETE and TRUNCATE.
It has the regression test that confirms that INSERT, UPDATE and DELETE
fail on its foreign table, but not TRUNCATE yet. It's better to
also test TRUNCATE fails on a foreign table not allowing TRUNCATE,
for test coverage. This commit adds that regression test using file_fdw.

Author: Yugo Nagata
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/20220630104812.ec1556481452c019874f4ac9@sraoss.co.jp

contrib/file_fdw/sql/file_fdw.sql

@@ -166,6 +166,7 @@ SELECT tableoid::regclass, b FROM agg_csv;
 INSERT INTO agg_csv VALUES(1,2.0);
 UPDATE agg_csv SET a = 1;
 DELETE FROM agg_csv WHERE a = 100;
+TRUNCATE agg_csv;
 -- but this should be allowed
 SELECT * FROM agg_csv FOR UPDATE;