Fix test AsyncUDPSocketTest for stress testing
Fix test AsyncUDPSocketTest for stress testing

Summary:
If multiple instances of this test are running, it is possible for a
filename conflict to occur. This fixes that behavior by using a different temp
name each time.

Reviewed By: yfeldblum

Differential Revision: D15258558

fbshipit-source-id: 40e71c57e68a90bc730fb5ba427d999789f6f88c

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly