Detect format-string mistakes in the libpq_pipeline test module.

Detect format-string mistakes in the libpq_pipeline test module.

I happened to notice that libpq_pipeline's private implementation
of pg_fatal lacked any pg_attribute_printf decoration.  Indeed,
adding that turned up a mistake!  We'd likely never have noticed
because the error exits in this code are unlikely to get hit,
but still, it's a bug.

We're so used to having the compiler check this stuff for us that
a printf-like function without pg_attribute_printf is a land mine.
I wonder if there is a way to detect such omissions.

Back-patch to v14 where this code came in.