Please, add async support for iteration setup and cleanup

[quixoticaxis]

It does not seem that [IterationSetup] and [IterationCleanup] may mark async methods for now. The code is failing with …the wrong return type error. Is there any reason why global counterparts are allowed to be async, while iteration bound methods are not?

[timcassell]

Support for that will be added if/when #2111 is merged.