fix sys.path earlier to make outputcapture work

We need to adjust sys.path earlier in test-webkitpy's execution so
that we can safely import unittest2 when configuring the printer.

Without this patch, running test-webkitpy -p could fail if unittest2
wasn't an alias for unittest (in Python 2.7).

R=ojan@chromium.org, abarth@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/22876016

git-svn-id: svn://svn.chromium.org/blink/trunk@156357 bbb929c8-8fbe-4397-9dbb-9b2b20218538