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.
