Complete product functional tests

I complete implementing the remaining functional tests for products.  I
introduces the bug fixes of the last commit into these test cases and
fix some new bugs:

- pricing instances were not deleted recursively when deleting sales
  items
- transfer quantity, unit, and unit price when removing pricing to the
  input fields in the first fieldset
- fix value of input field "relToPos" and the user-visible position
  only if the row is of type "relativeToPos" when removing rows; this
  produced values of -1 in relToPos and input errors when submitting
  the form