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/Config/DatabaseTest.php

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

tests/unit/Models/ModelValidatorTest.php

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

tests/unit/Models/NewsletterOptionTest.php

@@ -8,6 +8,7 @@ class NewsletterOptionTest extends MailPoetTest {
   public $data;
 
   function __construct() {
+    parent::__construct();
     $this->data = array(
       'newsletter_id' => 1,
       'option_field_id' => 2,

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,

tests/unit/Newsletter/AutomatedLatestContentTest.php

@@ -3,6 +3,7 @@ use MailPoet\Newsletter\AutomatedLatestContent;
 
 class AutomatedLatestContentTest extends MailPoetTest {
   function __construct() {
+    parent::__construct();
     $this->alc = new AutomatedLatestContent();
   }
 

tests/unit/Newsletter/RendererTest.php

@@ -13,6 +13,7 @@ use MailPoet\Newsletter\Renderer\Renderer;
 
 class NewsletterRendererTest extends MailPoetTest {
   function __construct() {
+    parent::__construct();
     $this->newsletter = array(
       'body' => json_decode(
         file_get_contents(dirname(__FILE__) . '/RendererTestData.json'), true

tests/unit/Newsletter/ViewInBrowserTest.php

@@ -11,6 +11,7 @@ use MailPoet\Router\Router;
 
 class ViewInBrowserTest extends MailPoetTest {
   function __construct() {
+    parent::__construct();
     $this->newsletter =
       array(
         'body' => json_decode(

tests/unit/Router/RouterTest.php

@@ -10,6 +10,7 @@ class FrontRouterTest extends MailPoetTest {
   public $router;
 
   function __construct() {
+    parent::__construct();
     $this->router_data = array(
       Router::NAME => '',
       'endpoint' => 'mock_endpoint',

tests/unit/Subscribers/ImportExport/Import/MailChimpTest.php

@@ -4,6 +4,7 @@ use MailPoet\Subscribers\ImportExport\Import\MailChimp;
 
 class MailChimpTest extends MailPoetTest {
   function __construct() {
+    parent::__construct();
     $this->api_key = getenv('WP_TEST_IMPORT_MAILCHIMP_API');
     $this->mailchimp = new MailChimp($this->api_key);
     $this->lists = explode(",", getenv('WP_TEST_IMPORT_MAILCHIMP_LISTS'));

tests/unit/Util/ConflictResolverTest.php

@@ -6,6 +6,7 @@ class ConflictResolverTest extends MailPoetTest {
   public $wp_filter;
 
   function __construct() {
+    parent::__construct();
     $this->conflict_resolver = new ConflictResolver();
     $this->conflict_resolver->init();
     global $wp_filter;