Fix discovery and running of tests in Visual Studio Test Explorer

[dmalovric]

Made some changes to the integrated example so that test discovery works for both CLI and Test Explorer in Visual Studio:

• Removed <OutputType>Exe</OutputType> as it is technically not necessary (but can be reintroduced if preferred)
• Set <DisableTestingPlatformServerCapability>true</DisableTestingPlatformServerCapability> to address known issues with XUnit and VS Test Explorer
• Added reference to Microsoft.NET.Test.Sdk as it's needed for Visual Studio Test Explorer
• Added reference to xunit.runner.visualstudio as it's needed for Visual Studio Test Explorer
• Created a small TestDiscoveryHandler that switches between:
  ConsoleRunner.Run(args);
  and
  TestPlatformTestFramework.RunAsync(args, SelfRegisteredExtensions.AddSelfRegisteredExtensions);
• Change Program.cs to call TestDicoveryHandler.IsTestRun and run the appropriate test runner if this is a test run:

if (TestDiscoveryHandler.IsTestRun(args, out var testRunner))
  return await testRunner();