public function test_foreach() {
    $callback_one  = '__return_null';
    $callback_two  = '__return_false';
    $hook          = new WP_Hook();
    $tag           = __FUNCTION__;
    $priority      = rand( 1, 100 );
    $accepted_args = rand( 1, 100 );

    $hook->add_filter( $tag, $callback_one, $priority, $accepted_args );
    $hook->add_filter( $tag, $callback_two, $priority + 1, $accepted_args );

    $functions  = array();
    $priorities = array();
    foreach ( $hook as $key => $callbacks ) {
      $priorities[] = $key;
      foreach ( $callbacks as $function_index => $the_ ) {
        $functions[] = $the_['function'];
      }
    }
    $this->assertSameSets( array( $priority, $priority + 1 ), $priorities );
    $this->assertSameSets( array( $callback_one, $callback_two ), $functions );
  }