Rather then using a sleep in the fake DB, go to a channel
select and wait for the context to be done.

Fixes #18115

Change-Id: I6bc3a29db58c568d0a7ea06c2a354c18c9e798b2
Reviewed-on: https://go-review.googlesource.com/33712
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>