Use the supported way to mark layers.

  - Don't call mark layer by default but provide helper method for the
    doctest classes to call on demand.
Removed the form specific test helpers.

  - Those helper classes have been moved to pmr2.z3cform, as that is the
    package dedicated to dealing with forms.
  - This package still provides certain helper classes and data that is
    needed to fully test PMR2 proper.
Changelog.
Removed deprecated import.
Changelog.