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

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/event/events_test.php because it uses
some renamed (core_calendar_externallib_testcase => \core_calendar\externallib_test)
classes.