Add tests for libpq gssencmode and sslmode options

Add tests for libpq gssencmode and sslmode options

Test all combinations of gssencmode, sslmode, whether the server
supports SSL and/or GSSAPI encryption, and whether they are accepted
by pg_hba.conf. This is in preparation for refactoring that code in
libpq, and for adding a new option for "direct SSL" connections, which
adds another dimension to the logic.

If we add even more options in the future, testing all combinations
will become unwieldy and we'll need to rethink this, but for now an
exhaustive test is nice.

Author: Heikki Linnakangas, Matthias van de Meent
Reviewed-by: Jacob Champion
Discussion: https://www.postgresql.org/message-id/a3af4070-3556-461d-aec8-a8d794f94894@iki.fi