Set cookie to `/` path to please headless drivers

In headless drivers based on MinkBrowserKitDriver, like MinkGoutteDriver, the cookie manipulation code forces root path (`/`) to all cookie operations (set/get). This doesn't create any problems if only driver methods are used to work with cookies.

However, when in `testCookie` a cookie is set from PHP via `setcookie('srvr_cookie', 'srv_var_is_set');`, then according to http://php.net/manual/en/function.setcookie.php the path isn't `/` by default (as it is with MinkBrowserKitDriver), but is path from url to a file, where a cookie was set from.

Because of that cookie set by PHP from a script in a sub-folder can't be replaced/removed via MinkBrowserKitDriver `setCookie` method.

Actually MinkZombieDriver also stores cookie at `/` path, but when it comes to delete a cookie, then it removes it from all paths and that's why this test passes for it.