[EPIC] Xunit 3 support

[Arkatufus]

Some TestKit implementation would need to have XUnit 3 support:

• Akka.Persistence.TestKit
• Akka.Hosting.TestKit

[Aaronontheweb]

Do we need to do the MNTR - and what about our internal usage of Xunit?

[Aaronontheweb]

We can do that later of course, but just asking about scope

[Aaronontheweb]

All of the packages that are not explicitly named Xunit[Version] - they should probably default to xUnit3.

How big of a pain is it to just upgrade everything to xUnit3?