Don't assume goroutines are gone immediately.

Copy the waitCondition & waitErrCondition helpers from net/http's
tests, so we can assume both exist in both repos, even though we only
use one of them as of this CL.

Fixes golang/go#22889

Change-Id: Ife251c9552bc68646e174a7ac082b363e32132a1
Reviewed-on: https://go-review.googlesource.com/114012
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>