Additions:
  * Remove PYTHONUNBUFFERED for svn tests

This obviates https://codereview.chromium.org/2298863002/.

recipe_engine:
  https://crrev.com/09345f9dcf4d2744f66d64766fd349ddf04f04ce Cancel PYTHONUNBUFFERED when unbuffered is explicitly set to False (iannucci@chromium.org)

TBR=martiniss@chromium.org, phajdan.jr@chromium.org
BUG=635641

Review-Url: https://codereview.chromium.org/2298893002