Teach test.py to add appengine paths to PYTHON_PATH
Teach test.py to add appengine paths to PYTHON_PATH

This makes it possible to run appengine tests
without the tests having to mangle sys.path themselves.

This uses very similar code for what is in PRESUBMIT.py:
https://codereview.chromium.org/517203002/
The major difference being that this code can use
import os directly instead of having to use input_api.

BUG=407734

Review URL: https://codereview.chromium.org/546583002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra