Update tests related projects

[filzrev]

This PR intended to update xUnit version to latest v3.
And switch test runner to use Microsoft.Testing.Platform based runner instead of VSTest.

What's changed in this PR.

1. Update test projects to use xUnit.v3 (5247872)

• 1.1. Update following NuGet packages and fix build errors.
  • Shouldly
  • Microsoft.NET.Test.Sdk
  • xunit -> xunit.v3
  • xunit.runner.visualstudio
• 1.2. Fix build errors/warnings.

2. Fix test code (0259c36)

• 2.1. Add tests\Directory.Build.props file and add setting to use Microsoft.Testing.Platform
• 2.2. Modify CSharpGeneratorRunner.cs and add additional DLL references.
  (It's seems be required when using xunit.runner.visualstudio v2.5.0 or later)
• 2.3. Implement IDisposable interface on some class and clear Environment.ExitCode on every test run finished.
• 2.4. Modify assembly version check logics at HelpTest.cs

3. Modify GeneratorSandbox project launch settings (c4a86c0)

[neuecc]

Thank you, very helpful!
I've also been trying v3 and Microsoft.Testing.Platform in a different repository recently, and I thought they looked good.