No need to test so many sizes in race mode, especially for a package
which doesn't use goroutines.

Reduces test time from 2.5 minutes to 25 seconds.

Updates #17104

Change-Id: I7065b39273f82edece385c0d67b3f2d83d4934b8
Reviewed-on: https://go-review.googlesource.com/29163Reviewed-by: David Crawshaw <crawshaw@golang.org>