ThatAsync and MultipleAsync

[uecasm]

Because #2843 and #3432 made me sad.

This introduces Assert.ThatAsync (which accepts Is or Throws constraints as usual) and Assert.MultipleAsync (which can contain both ThatAsync and synchronous assertions), and does all the right things with them (at least as far as the test method boundary), provided that you await them inside an async Task test.

Not included, but I was pondering whether it might be a good idea: constraint Is.Canceled and/or Throws.Canceled which is equivalent to Throws.InstanceOf<OperationCanceledException>().

[uecasm]

@dotnet-policy-service agree

[OsirisTerje]

@stevenaw I have read through the earlier discussions on this issue, and the implementation here looks very straightforward. I can't see any side-effects nor breaking changes here. Do you agree?

@jnm2 You had a long thread on this topic many years ago. Do you agree this PR looks ok?

[OsirisTerje]

LGTM
And this is additions, nothing existing is changed, afaic see.