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/ModelValidatorTest.php

@@ -6,6 +6,7 @@ class ModelValidatorTest extends MailPoetTest {
   public $validator;
 
   function __construct() {
+    parent::__construct();
     $this->validator = new ModelValidator();
   }