Simplify run-bindings-test

A bit more simplification, while we're at it.
We don't need a class for the tests: we're just calling a function once.
This removes a lot of OO boilerplace (lots of 'self') and makes the code
very functional
(an object with a main() method is a code smell).
Also simplify path handling (we already know Source/ for Python includes,
so we don't need to ask the source control system for it).

R=haraken
BUG=341748

Review URL: https://codereview.chromium.org/216663004

git-svn-id: svn://svn.chromium.org/blink/trunk@170323 bbb929c8-8fbe-4397-9dbb-9b2b20218538