Fixes the darwin/arm builder, which has a special test runner which
makes the assumption that tests never use testdata from another
package.

This looks large, but it's no more space in git.

Change-Id: I81921b516443d12d21b77617d323ddebedbe40f8
Reviewed-on: https://go-review.googlesource.com/21907Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>