This adds the ability for the group_manager method to take additional
keyword arguments that are passed to the underlying Test constructor.
This allows groups that all need the same arguments (say platform
requirements), but be passed to the constructor and be added by the
adder.

>>> prof = TestProfile()
>>> with prof.group_manater(Test, 'group', required_platforms=['glx']) as g:
...     g(['glx-foobar'])
>>> prof.test_list['group/glx-foobar'].required-platforms == ['glx']
True