Waiting 2ms for all the kicked-off goroutines to run and block
seems a little optimistic. No harm done by waiting for 200ms instead.

Fixes #17238.

Change-Id: I827532ea2f5f1f3ed04179f8957dd2c563946ed0
Reviewed-on: https://go-review.googlesource.com/32103
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>