Tests for psql \g and \o

Hi,

Here's a patch adding regression tests for \g and \o, and TAP tests
for \g | program,

It's a follow up to the discussion at [1]. Since this discussion
already has a slot in the CF [2] with a committed patch, let's start a
new separate thread.

[1]
https://www.postgresql.org/message-id/4333844c-2244-4d6e-a49a-1d483fbe304f@manitou-mail.org

[2]  https://commitfest.postgresql.org/40/3923/


Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite