Plone 4.3 tests: remove collective.z3cform.wizard from auto-checkout.

The change was released in 1.4.8.
Refactor test setup and use ftw.builder / ftw.testbrowser.

The p.a.testing layers (testing.py) were not yet used and could not
have been working.
Add browser tests for shopping cart portlet.
Add browser tests for checkout.