test(ui5-dynamic-date-range): refactor and stabilize tests (#12555)

Stabilized the following tests:

- Next X Weeks: Moved initialization from onPopoverOpen to onPopoverBeforeOpen to prevent late value resets after selection and ensure consistent date range calculation.

- FromDateTime and ToDateTime: Tests now assert behavior independent of the current month.

Also made some small code style changes.

Fixes: #12601, #12536

Author: didip1000