Date helper days_in_month(), CI_Calendar::get_total_days() changes

 - days_in_month() now uses cal_days_in_month(), if available.
 - CI_Calendar::get_total_days() is now an alias for days_in_month().
[ci skip] Fix date helper loading in CI_Calendar::get_total_days()
Fix CI_Calendar tests
Remove a few empty 'mock' classes