Currently the benchmarks hard code "10" for the cost but maybe a
better benchmark would be to use the DefaultCost, since this is the
most common use mode for the bcrypt package.

Change-Id: Ie89853fcbe4a3a2830fff812ae5cfe908df145fe
Reviewed-on: https://go-review.googlesource.com/c/144338Reviewed-by: Filippo Valsorda <filippo@golang.org>