Add tests about the RuntimeError issue

when HTTPretty.enable(), we should contine to do DNS query,
like it's done in the
test_httpretty_should_not_raise_on_socket_send_when_uri_not_registered
test.

The test test_httpretty_should_raise_on_socket_send_when_uri_registered
is written to check that the RuntimeError is still raised when
the port is used for http traffic.