Implement TestKitBase and Xunit.TestKit Breaking changes

- Most of TestKitBase is implemented
- Added a TestKit for xUnit.
- Moved AkkaSpec out of TestKit, It's for internal testing and is not part of TestKit
- ActorSystem contains a AwaitTermination that can be used for waiting for system shutdown
- Other changes are due to breaking changes in TestKit (Making public members PascalCase, and renaming)