Automated testing strategy and examples

The documented strategy and guide to writing tests are being committed into the repository. Here are the links:

• Automated testing strategy - https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/blob/testing/docs/testing-strategy.md
• Writing tests - https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/blob/testing/docs/writing-tests.md

This MR seems large but contains almost no production code changes. Most added lines are in package-lock.json and in the new JSON fixtures mocking the API responses.

Test examples:

After you read the documents, you can see the following examples:

• unit test - git_remote_parser.test.js (original test made into unit test)
• integration test - tree_view.test.js

If I had more time, I would

1. Write example integration test for command
   • Create test helper that would spy on vscode commands in integration tests, maybe we'll have to create a mocked command registry
2. Write example integration test for the status bar
3. Write one unit test for the webview project
4. Create more examples of uni tests, even though those are the most straightforward.

Closes #199 (closed)