Port more TestHost functionality

#65
TODO: #66 - `TestServer.Create<TStartup>`
TODO: Remove the placeholder TestClient, reconcile with HttpClient and ClientHandler. Wait until MVC tests transition to HttpClient. 

Eng guidelines.
"Unit test class naming 
Test class names end with “Test” and live in the same namespace as the class being tested. For example, the unit tests for the Microsoft.Fruit.Banana class would be in a Microsoft.Fruit.BananaTest class in the test assembly."