Add .ThrowsAsync() that will correctly mock exception on async methods

[Socolin]

It should fix #609

[Socolin]

I'm unsure about some part, I had to add #if for net4.5 (no Task.FromException)
Also netstandard < 2.1 and netcoreapp < 2.1 does not have ValueTask

I did use some reflection to find the appropriate Task.FromException / Task.FromException<T> when using .ThrowsAsync<TException>() on a Task<TResult> to avoid to force the users to specify the return type each time (.ThrowsAsync<TResult, TException>()) but I'm not sure about this part. Since I was not able to do the same for ValueTask<T>, if you have any preference on this let me know I can change it.

[dtchepak]

Thanks a lot for this! It looks great!

I'm playing around with some ideas for reducing the scope of the #if switches. I'll get back to you.

[Socolin]

I've updated the PR with your changes

[dtchepak]

Thanks @Socolin ! Sorry for the delay in merging and my incomplete PR 🤦

[Socolin]

Thanks @Socolin ! Sorry for the delay in merging and my incomplete PR facepalm

That's great news :) Thanks to have taken time for this.