Fix unit tests for PHPUnit v6

Codeception from version 2.3 up comes with PHPUnit v6 which changed
__construct behaviour. Our tests have to call parent __constructor in
order to work. The error was:
[PHPUnit\Framework\Exception] array_merge(): Argument #1 is not an array

tests/unit/Models/SubscriberCustomFieldTest.php

@@ -3,6 +3,7 @@ use MailPoet\Models\SubscriberCustomField;
 
 class SubscriberCustomFieldTest extends MailPoetTest {
   function __construct() {
+    parent::__construct();
     $this->data = array(
       array(
         'custom_field_id' => 10,