In normal mode the test runs for 9+ seconds on my machine (48 cores).
But the real problem is race mode, in race mode it hits 10m test timeout.
Reduce test size in short mode. Now it runs for 100ms without race.

Change-Id: I9493a0e84f630b930af8f958e2920025df37c268
Reviewed-on: https://go-review.googlesource.com/19956Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>