Fix TrackerTest provideContainerConfig() method. Now that it is called during...

Fix TrackerTest provideContainerConfig() method. Now that it is called during test case setup, check the const is defined before defining it. Otherwise, there will be an error regarding redefining the constant.

tests/PHPUnit/System/TrackerTest.php

@@ -308,7 +308,9 @@ class TrackerTest extends IntegrationTestCase
 
     public function provideContainerConfig()
     {
-        define('DEBUG_FORCE_SCHEDULED_TASKS', 1);
+        if (!defined('DEBUG_FORCE_SCHEDULED_TASKS')) {
+            define('DEBUG_FORCE_SCHEDULED_TASKS', 1);
+        }
 
         $testingEnvironment = new \Piwik_TestingEnvironment();