After upgrading builder device (android/arm) to android 5.0.2,
the test started failing. Running 'ln -s' from shell fails with
permission error.

Change-Id: I5b9e312806d58532b41ea3560ff079dabbc6424e
Reviewed-on: https://go-review.googlesource.com/22962Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>