class TestMailManager

class TestMailManager extends MailManager {
    
    /**
     * Sets the discovery for the manager.
     *
     * @param \Drupal\Component\Plugin\Discovery\DiscoveryInterface $discovery
     *   The discovery object.
     */
    public function setDiscovery(DiscoveryInterface $discovery) {
        $this->discovery = $discovery;
    }
    
    /**
     * {@inheritdoc}
     */
    public function doMail($module, $key, $to, $langcode, $params = [], $reply = NULL, $send = TRUE) {
        // Build a simplified message array and return it.
        $message = [
            'id' => $module . '_' . $key,
            'module' => $module,
            'key' => $key,
            'to' => $to,
            'from' => 'from@example.org',
            'reply-to' => $reply,
            'langcode' => $langcode,
            'params' => $params,
            'send' => TRUE,
            'subject' => '',
            'body' => [],
        ];
        return $message;
    }

}