Bug 966516 - [B2G][Calender]Changing the month in an event doesn't update to the number associated with that month r=gaye

I wrote only unit tests (disabled because of intermittent failures on other tests), since there is no way to write marionette tests for this bug (we can't set the system time on a reliable way - b2g always use the host clock).

I also decided to split the method into many methods to make it easier to test. - easier to inject the `targetElement` and don't need to trigger the event.