[WIP][ticket/11579] Add basic set of unit tests for validate_data()

This currently includes tests for the string, num, date, match, and
language iso validation functions.

More tests will be added as soon as possible.

The currently added comments regarding the tests input, settings, and expected results are only for tracking the specific cases during writing the tests. They will be removed later.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11579

PHPBB3-11579