MDL-25505 behat: Adding a time gap between two cron runs.

According to \core\task\scheduled_task::get_next_scheduled_time() we
wait 61 seconds, as the next scheduled time is in the next minute.
MDL-44358 simplify CSS processing

The affected code areas are not supposed to be used in any add-ons,
the backwards compatibility should not be a concern for us.