file_fdw: Add regression test for file_fdw with ON_ERROR='set_null'

file_fdw: Add regression test for file_fdw with ON_ERROR='set_null'

Commit 2a525cc97e1 introduced the ON_ERROR = 'set_null' option for COPY,
allowing it to be used with foreign tables backed by file_fdw. However,
unlike ON_ERROR = 'ignore', no regression test was added to verify
this behavior for file_fdw.

This commit adds a regression test to ensure that foreign tables using
file_fdw work correctly with ON_ERROR = 'set_null', improving test coverage.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Yi Ding <dingyi_yale@163.com>
Discussion: https://postgr.es/m/CAHGQGwGmPc6aHpA5=WxKreiDePiOEitfOFsW2dSo5m81xWXgRA@mail.gmail.com