This change merges unicast_posix_test.go and multicast_test.go into
listen_test.go before deflaking tests for Listen functions.

No code changes.

Change-Id: Ic4cd6531b95dfb5b6e6e254241692eca61a71e94
Reviewed-on: https://go-review.googlesource.com/9460Reviewed-by: Ian Lance Taylor <iant@golang.org>