To reproduce:
1. Download and open users attached project Network test
2. Open scene "server"
3. Build and run this scene
4. While running built project press play in Unity editor

Expected result: Error "Cannot open socket on ip {*} and port {8888}; check please your network, most probably port has been already occupied"
Actual result: No errors

Regression appeared in 5.6.0b3
Reproducible on 5.6.0b3 till 5.6.0p1, 2017.1.0b1
Not reproducible 5.5.0f3, 5.6.0b1

Note: On not reproducible versions error "Cannot open socket on ip {*} and port {8888}; check please your network, most probably port has been already occupied" occurs.