How is a separate base class better? :)

Having a default coroutine runner on the standard TestCase means all you have to do to add async tests is tack an `async` onto the front of your method definition, and everything just works as it always has.  Making it a separate base class means you have to know about that base class, what it's called, and remember to derive your test class from it.  If you accidentally add `async` to the front of a test method in a TestCase-derived test class, you get mostly-silent success with an easily-ignored warning about a coroutine not being awaited.