runtime: deflake TestCgoCheckBytes

Bump up the multiplier to 20.  Also run the fast version first, so that
the slow version is likely to start up faster.

Change-Id: Ia0654cc1212ab03a45da1904d3e4b57d6a8d02a0
Reviewed-on: https://go-review.googlesource.com/19835


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/22070


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>