Fix the TestServer overloads

[davidfowl]

They are a mess https://github.com/aspnet/Hosting/blob/dev/src/Microsoft.AspNet.TestHost/TestServer.cs#L36-L96. We need to change this API to look more like WebHostBuilder so it can be configured without adding overloads.

[davidfowl]

/cc @muratg @Tratcher

[Tratcher]

Maybe this can consolidate down to something like:

var server = TestServer.Create(webHostBuilder =>
{
  webHostBuilder.UseStartup<MyStartup>();
});

[JunTaoLuo]

Will be addressed by #525