Use Vscode Integration Test Setup

[tricktron]

Microsoft has reworked the testing setup to integrate integration tests more easily:

• https://github.com/microsoft/vscode-test
• https://code.visualstudio.com/api/working-with-extensions/testing-extension#migrating-from-vscode

Let's consider using it.

[tricktron]

After some time of consideration, I still think that the effort is not worth it. Closed for now.