add helper method get authentication

This allow to create authenticated client with :

```
use Chill\MainBundle\Tests\TestHelper as MainTestHelper;

class XYZTest extens WebTestCase {

   public function testABC()
   {
      $client = static::createClient(array(),
              MainTestHelper::getAuthenticatedClientOptions()
              );

      //...
   }

}

```

Tests/TestHelper.php

@@ -0,0 +1,46 @@
+<?php
+
+/*
+ * Chill is a software for social workers
+ * Copyright (C) 2015 Champs Libres <info@champs-libres.coop>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace Chill\MainBundle\Tests;
+
+use Symfony\Component\BrowserKit\Client;
+
+/**
+ * Provide useful methods for tests
+ *
+ * @author Julien Fastré <julien.fastre@champs-libres.coop>
+ * @author Champs Libres <info@champs-libres.coop>
+ */
+class TestHelper
+{
+    /**
+     * create a client authenticated with an user
+     * 
+     * @param WebTestCase $testCase
+     * @return \Symfony\Component\BrowserKit\Client authenticated client
+     */
+    public static function getAuthenticatedClientOptions()
+    {
+       return array(
+           'PHP_AUTH_USER' => 'center a_social',
+           'PHP_AUTH_PW'   => 'password',
+        );
+    }
+}