Rework functional tests

I make some major modifications to the functional tests to reflect the
coding changes from the last commits:

- Check unified display of closing balances and still unpaid values.
- Check display of cascading validation errors.
- Fix errors in functional tests.
- Swap two fields in the show view of sales orders to be conform to the
  field order in the form.
- Unify tests of created shipping dates.
- Fix rounding issues which produced values such as "-0,00".
- Fix import error in UserServiceTests.