DateCalendar: add unit tests (#103676)

* Simplify types around required

* Install test utilities

* Set jest timezone to UTC

* Add DateCalendar unit tests

* Move test utils file as it errors in CI

* Explain better last timezone test

* Remove need for `date-fns-tz` dependency

* .toBeInTheDocument() => .toBeVisible()

* Do not export internal utility function

* Add selected disabled dates tests

Author: ciampo