add ability to nest test suites

It is nice to be able to group all tests of certain types together, it makes viewing and finding tests easier. (Granted, having tags is useful too, when wanting to associate tests multiple ways across test suites - but that is a different feature request.)

For example, I have two test suites, one of positive test using the payment action and another of negative tests. It would be nice to group both test suites into an enclosing test suite of payment tests and then be able to do this for each type of action.