Add some test coverage for ecpg's "long long" support.

This will only actually exercise the "long long" code paths on platforms
where "long" is 32 bits --- otherwise, the SQL bigint type maps to
plain "long", and we will test that code path instead.  But that's
probably sufficient coverage, and anyway we weren't testing either
code path before.

Dang Minh Huong, tweaked a bit by me

Discussion: https://postgr.es/m/151935568942.1461.14623890240535309745@wrigleys.postgresql.org