Revert switch to KernelTestCase for ContentProxyTest

Define the error string manually inside the test class instead of
fetching it from app config.

tests/Wallabag/CoreBundle/CoreKernelTestCase.php

@@ -1,17 +0,0 @@
-<?php
-
-namespace tests\Wallabag\CoreBundle;
-
-use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
-
-abstract class CoreKernelTestCase extends KernelTestCase
-{
-    protected $fetchingErrorMessage;
-
-    public function setUp()
-    {
-        self::bootKernel();
-        $container = self::$kernel->getContainer();
-        $this->fetchingErrorMessage = $container->getParameter('wallabag_core.fetching_error_message');
-    }
-}

tests/Wallabag/CoreBundle/Helper/ContentProxyTest.php

@@ -1,16 +1,17 @@
 <?php
 
-namespace tests\Wallabag\CoreBundle\Helper;
+namespace Tests\Wallabag\CoreBundle\Helper;
 
 use Psr\Log\NullLogger;
 use Wallabag\CoreBundle\Helper\ContentProxy;
 use Wallabag\CoreBundle\Entity\Entry;
 use Wallabag\CoreBundle\Entity\Tag;
 use Wallabag\UserBundle\Entity\User;
-use Tests\Wallabag\CoreBundle\CoreKernelTestCase;
 
-class ContentProxyTest extends CoreKernelTestCase
+class ContentProxyTest extends \PHPUnit_Framework_TestCase
 {
+    private $fetchingErrorMessage = 'wallabag can\'t retrieve contents for this article. Please <a href="http://doc.wallabag.org/en/master/user/errors_during_fetching.html#how-can-i-help-to-fix-that">troubleshoot this issue</a>.';
+
     public function testWithBadUrl()
     {
         $tagger = $this->getTaggerMock();