Add integration tests

[MichaelCurrin]

This is a long term goal as the app has simple UI. See the notes in test section of the docs.

Note that there are different approaches to integration tests. The approach in semantic commit messages is nice. It uses vscode-test as a package in dev deps, which is downloaded when added to .vscode-test dir at the root.

I got error messages before that vscode tests would not run while vscode is running, but this seems okay now in vscode-git-semantic-commit when running my fork and vscode 1.48 downloaded.

Here is the approach in git-prefix to downloading and running vscode

		"postinstall": "node ./node_modules/vscode/bin/install",
		"test": "npm run compile && node ./node_modules/vscode/bin/test"

But the tests doesn't actually do much because of limited coverage.