playground/socket: deflake test, remove sleep

Fixes golang/go#18275

Change-Id: Iedb0a21a3fe0b383a2e51fc92076d7fd005d71e6
Reviewed-on: https://go-review.googlesource.com/34277
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed