Fix libpq_encryption tests when compiled without SSL support

Fix libpq_encryption tests when compiled without SSL support

It correctly skipped tests involving SSL in the server when SSL
support was not compiled in, but even when SSL is not enabled in the
server and the connection is established without SSL, libpq behaves
differently in many of the test scenarios when libpq is compiled
without SSL support. For example, with sslmode=prefer, if libpq is
compiled with SSL support it will attempt to use SSL, but without SSL
support it will try authenticating in plaintext mode directly. The
expected test output didn't take that into account.

Discussion: https://www.postgresql.org/message-id/CA%2BhUKG%2BHRTtB%2Bx%2BKKKj_cfX6sNhbeGuqmGxjGMwdVPG7YGFP8w@mail.gmail.com