fix9Datepicker tests (#807)

* test(date picker): wip - add tests for datepicker

* fix(datepicker): add basic tests for datepicker with default config

This PR adds tests for all functionality of the datepicker with
default config options. This includes keyboard navigation
and axe accessibility checks.

During testing a couple of small issues were found and fixed:
 * Date was initialized with an odd format order causing
issues during testing
 * Calculation of the first and last day of the week was
incorreect when the last day of the week was a Sunday