tests/qtest/netdev-socket: Do not test multicast on Darwin

tests/qtest/netdev-socket: Do not test multicast on Darwin

Do not run this test on Darwin, otherwise we get:

  qemu-system-arm: -netdev dgram,id=st0,remote.type=inet,remote.host=230.0.0.1,remote.port=1234:
   can't add socket to multicast group 230.0.0.1: Can't assign requested address
  Broken pipe
  ../../tests/qtest/libqtest.c:191: kill_qemu() tried to terminate QEMU
   process but encountered exit status 1 (expected 0)
  Abort trap: 6

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230918062549.2363-1-philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>