Support running tests multiple times under different cultures

Sometimes a test or fixture needs to be run under various cultures. The culture can be changed using the SetCulture attribute, but that is only allowed once per test. It would be desireable to allow the attribute to be used multiple times, or with a comma-separated list of cultures.

Further notes:
1. 'Culture' is probably preferable to SetCulture as an attribute.
2. Both separate attributes and a comma-separated version should be supported.
3. UI Culture should be settable in the same way, either separately or in conjunction with culture.