MDL-39474 introduce $CFG->debugdeveloper and cleanup $CFG init

lib/phpunit/classes/advanced_testcase.php

@@ -248,7 +248,8 @@ abstract class advanced_testcase extends PHPUnit_Framework_TestCase {
     }
 
     /**
-     * Clear all previous debugging messages in current test.
+     * Clear all previous debugging messages in current test
+     * and revert to default DEVELOPER_DEBUG level.
      */
     public function resetDebugging() {
         phpunit_util::reset_debugging();

lib/phpunit/classes/util.php

@@ -594,6 +594,7 @@ class phpunit_util extends testing_util {
      */
     public static function reset_debugging() {
         self::$debuggings = array();
+        set_debugging(DEBUG_DEVELOPER);
     }
 
     /**