Improve the group test factory

Generate a different name for each test group, and require the user who
will create the group as a param.

This will break some existing tests...
[#1526] Fix some tests broken by commit b117294987
Merge branch 'master' of github.com:ckan/ckan into improved-group-factory
Merge pull request #1526 from ckan/improved-group-factory

Improve the group test factory