Uninstall test: test that there is uninstall method.

Uninstall external methods are still necessary today for properly uninstalling a package.
See http://blog.keul.it/2013/05/how-to-make-your-plone-add-on-products.html
Generic Setup uninstall test: Remove is_product option

Remove is_product option since we require an uninstall
external method which requires the package to be a
product anyway.
Merge pull request #11 from 4teamwork/jone-uninstall-method

Uninstall test: test that there is uninstall method.