pgsql: Add regression test for TRUNCATE on foreign table not supporting

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ecc84b916f8f36924e2b3de358045edfc3205e5a

Modified Files
--------------
contrib/file_fdw/expected/file_fdw.out | 2 ++
contrib/file_fdw/sql/file_fdw.sql      | 1 +
2 files changed, 3 insertions(+)