sw test tool: support specifying the port number

You can specify the port number in the options when sw test tool is started.
The example is as follows:

  ryu-manager --test-switch-target_recv_port 4   (default:1)
              --test-switch-target_send_port_1 5 (default:2)
              --test-switch-target_send_port_2 6 (default:3)
              --test-switch-tester_send_port 7   (default:1)
              --test-switch-tester_recv_port_1 8 (default:2)
              --test-switch-tester_recv_port_2 9 (default:3)
               ryu/tests/switch/tester.py

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>
sw test tool: add a comment about specifying the port number support

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>
sw test tool: add unit test

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>