Add test cases for SocketUrlDetection

[fuji-151a]

There is no test class in SocketUrlDetection. So, I created SocketUrlDetectionTest class and I wrote the test cases. Please check this pull request.

Thanks.

[fuji-151a]

@erikdw Thank you for review. I rewrote code based on your review.

• modified constant name: PORT --> HOPEFULLY_UNUSED_PORT
• chaged port 9999 --> 29999
• added comment to isReachableFalse method

Please check again.

[erikdw]

@fuji-151a : thanks for the great comments that make it very clear what is happening.