UserMailerTest.php

<?php
      // In my test function 
      
      $this->Mailer->expects($this->once())
            ->method('onRegistration');

        $this->Mailer->expects($this->once())
            ->method('welcome');

      // ******************************
            
      // The Mailer Functions
      
      public function onRegistration(Event $event, Entity $entity, $options = [])
     {
            $var = '';
            if ($entity->isNew()) {
                  $this->send('welcome', [$entity]);
                  $this->send('registration_alert', [$entity]);
            }
     }
     
     public function welcome($user)
      {
             $this
                  ->to('[email protected]')
                  ->subject(sprintf('Welcome %s', $user->first_name));
      
            $this->set('user', $user);
      }