Adds testing prototype solution. Related #279

[Adam-it]

🎯 Aim

The aim is to prototype integration tests for the solution

To debug tests:

1. install extension https://marketplace.visualstudio.com/items?itemName=ms-vscode.extension-test-runner
2. run npm run package && npm run compile-tests
3. run Extension Tests in debug

To run:

1. run npm run package && npm run compile-tests
2. run npm run test and check the result in terminal

📷 Result

✅ What was done

• added test-cli and test-electron
• added a simple test that checks if extension is running

🔗 Related issue

Related #279