A test that needs 8mb of data doesn't look like a unit test to me.
Why you need 8mb, you can't test the same with a few bytes only?

I think that the problem is that some times is very useful to use the assertion framework and the test runner to make another kind of tests ("user story tests", "stress test", etc). But the SUnit framework depends on classification to detect all test cases available (unit tests or not).
When I run all the unit tests on the system, I expect unit tests, that is small and fast tests.

Maybe we can have a Test trait or something like that to reuse the assertion framework, and the test runners, and keep the TestCase hierarchy as a classification for unit tests.