Incorrect return type on kernel test route provider

This seems fine, though I assume the code in question is not used in any core tests, otherwise an error would have surfaced by now.

One q: RouteProviderInterface is documented like this:

/**
   * Returns all the routes on the system.
   *
   * Usage of this method is discouraged for performance reasons. If possible,
   * use RouteProviderInterface::getRoutesByNames() or
   * RouteProviderInterface::getRoutesByPattern() instead.
   *
   * @return \Symfony\Component\Routing\Route[]
   *   An iterator of routes keyed by route name.
   */
  public function getAllRoutes();

We should probably update that doc, either by replacing the array return type with iterator if it's safe, or maybe making it array or iterator. Can probably be done in a follow up though?

lgtm

Committed/pushed to main, thanks!