Extra features for coroutines (AKA "async def" methods as tests)

[clebergnu]

The most important parts #4767 have been implemented, but there are a couple of extra ideas/features:

• Allow wrappers (callables) to be used for special types of tests
• Have a default set of wrappers for "async" tests that will run the coroutine in a suitable environment (such as a new or default loop).

Let's evaluate and possible implement those.