Fix add permission

- By default, add permission should be granted to Site Administrator and Contributor roles, and not to Editor rol
- Use camel case on permission name as is the standard in Zope/Plone
Move upgrade tests to a new module
Refactor tests

Remove needless tests; by default, only Manager and Site Administrator roles have permission to add content on the root of a site. Also, the use cases are already covered by other tests. We want to test our code, not Plone's.