MDL-73485 phpunit: externallib, generator and filter tescase names

All externallib_test, generator_test and filter_test classes:

- Namespaced with component[\level2-API]
- Moved to level2-API subdirectory when required.
- Fixed incorrect use statements with leading backslash.
- Changed code to point to global scope when needed.
- Fix some relative paths and comments here and there.
- All them passing individually.
- Complete runs passing too.

Special mention to tests under testing/tests:

1) The core_testing component doesn't exist.
2) But testing/tests are allowed because there is a suite pointing to it (phpunit.xml).
3) So, the only possible namespace for them is "core".
4) And to avoid problems with other core testcases (under lib/tests)
   they have been renamed to have testing_xxxx as prefix.

Finally, also modified calendar/tests/events/events_test.php because it uses
some renamed (core_calendar_externallib_testcase => \core_calendar\externallib_test)
classes.

mod/folder/tests/generator_test.php

@@ -14,24 +14,17 @@
 // You should have received a copy of the GNU General Public License
 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 
-/**
- * mod_folder generator tests
- *
- * @package    mod_folder
- * @category   test
- * @copyright  2013 Marina Glancy
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
+namespace mod_folder;
 
 /**
- * Genarator tests class for mod_folder.
+ * Generator tests class for mod_folder.
  *
  * @package    mod_folder
  * @category   test
  * @copyright  2013 Marina Glancy
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class mod_folder_generator_testcase extends advanced_testcase {
+class generator_test extends \advanced_testcase {
 
     public function test_create_instance() {
         global $DB, $USER;
@@ -57,7 +50,7 @@ class mod_folder_generator_testcase extends advanced_testcase {
             'course' => $course->id,
             'files' => file_get_unused_draft_itemid()
         );
-        $usercontext = context_user::instance($USER->id);
+        $usercontext = \context_user::instance($USER->id);
         $filerecord = array('component' => 'user', 'filearea' => 'draft',
                 'contextid' => $usercontext->id, 'itemid' => $params['files'],
                 'filename' => 'file1.txt', 'filepath' => '/');