RecipientGatewayEvent::getGatewaysSorted does not return any gateways

From #2643408: Implement a more intelligent SMS Gateway selection mechanism for SMS Framework #53

There's a bug in this hunk

public function getGatewaysSorted() {
  $gateways = $this->gateways;
  uasort($gateways, function($a, $b) {
    list(, $priority_a) = $a;
    list(, $priority_b) = $b;
    if ($priority_a == $priority_b) {
      return 0;
    }
    return ($priority_a > $priority_b) ? -1 : 1;
  });

  $gateways = [];
  foreach ($gateways as $tuple) {
    list($gateway, ) = $tuple;
    $gateways[] = $gateway;
  }

  return $gateways;
}

Where $gateways = [] is initialized then iterated over. It also means we need tests for that class.