Faster bootstrap for PHPUnit tests

When running PHPUnit tests other than unit tests, the bootstrap checks that Piwik server is running correctly.

Instead of hitting the home page (`/` 302 redirect to `/index.php? CoreHome…`) which takes a couple seconds for the HTTP response, this uses a simpler and faster controller action.

The **TestsRunner** plugin is disabled in production right? So that new controller shouldn't be a problem.