Check in unittest2 so that we don't have to autoinstall it.

This involved adjusting test-webkitpy and a few unittest scripts to
ensure that webkitpy/thirdparty is always inserted into sys.path (both
unittest2 and coverage need to be installed somewhere in sys.path).

Also, I suspect autoinstalling unittest2 has been broken for a while
(we don't notice it on Precise or MountainLion).

TBR=eseidel@chromium.org, ojan@chromium.org
BUG=230706

Review URL: https://codereview.chromium.org/19688005

git-svn-id: svn://svn.chromium.org/blink/trunk@154467 bbb929c8-8fbe-4397-9dbb-9b2b20218538