test: make sure tests don't hang if default connection points to a closed port

test: make sure tests don't hang if default connection points to a closed port

They might hang is the server doesn't accept connections on the standard
port.

Tested by forcing the default connection to go to an unresponsive port,
using:

    export PGHOST=192.0.2.1
    export PSYCOPG_TEST_DSN="host=localhost dbname=psycopg_test"

and fixing all the tests hanging because they try to connect to the
unroutable hostname, not taking into account the test dsn.