Add a test for "cannot truncate foreign table"

Hello,

During checking regression tests of TRUNCATE on foreign
tables for other patch [1], I found that there is no test
for foreign tables that don't support TRUNCATE. 

When a foreign table has handler but doesn't support TRUNCATE,
an error "cannot truncate foreign table xxx" occurs. So, what
about adding a test this message output? We can add this test
for file_fdw because it is one of the such foreign data wrappers.

I attached a patch.

[1] https://postgr.es/m/20220527172543.0a2fdb469cf048b81c0967d3@sraoss.co.jp

-- 
Yugo NAGATA <nagata@sraoss.co.jp>