Add Orleans.Transaction.Testkit project structure

[xiazen]

This pr introduced the project struct for Orleans.Transaction.Testkit. It added two projects to src folder

• Orleans.Transaction.TestkitBase, which should contain all the test runners. But this project isn't using any specific test framework. It has a simple test runner TransactionalStateStorageTestRunner.
• Orleans.Transaction.TestKit.Xunit. This project uses Xunit as its test framework, it depend son Orleans.Transaction.TestkitBase, and it also has identical test runners with Orleans.Transaction.TestkitBase. Those test runner simply inherit the base test runners, but configured with Xunit specific Test annotation.

It is set up this way because there's multiple test frameworks, which c# programmers are using. Each test framework has its own specific test annotations. I don't want to limit user to only use Xunit. I want users to be able to potentially use other test frameworks at their choice. That's why there's TestkitBase project, which isn't any test framework specific. And there's a Testkit.Xunit project which uses Xunit. If a user want to use Xunit to write tests, they should uses Orleans.Transaction.TestKit.Xunit nuget. If they want to use other test framework, we can add a new project/nuget built for that test framework, or they can build it themselves.