nacl is really giving a hard time. avoid all external dependencies in the test.
Worked with trybots, failed in the build. No explanation, but this should fix it.

TBR=rsc

Change-Id: Icb644286dbce88f17ee3d96ad90efba34a80a92d
Reviewed-on: https://go-review.googlesource.com/11291Reviewed-by: Rob Pike <r@golang.org>