testing/test_env.py: don't silently disable the sandbox

If the sandbox is not installed, don't silently disable it.

BUG=245376
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/17648002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@208512 4ff67af0-8c30-449e-8e8b-ad334ec8d88c