ecpg: Add TAP test for the ecpg command.

ecpg: Add TAP test for the ecpg command.

This commit adds a TAP test to verify that the ecpg command correctly
detects unsupported or disallowed statements in input files and reports
the appropriate error or warning messages.

This test helps catch bugs like the one introduced in commit 3d009e45bd,
which broke ecpg's handling of unsupported COPY FROM STDIN statements,
later fixed by commit 94b914f601b.

Author: Ryo Kanbayashi <kanbayashi.dev@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CANOn0EzoMyxA1m-quDS1UeQUq6FNki6+GGiGucgr9tm2R78rKw@mail.gmail.com