Add tests and test plan for GitHub database download

[koesie10]

This adds some simple test for promptGitHubRepositoryDownload that just check the logic within this method and mocks out all other calls since these are already tested separately. It also adds a step to the test plan for testing the basic functionality. Unfortunately, it's hard to test the update functionality without waiting for a new commit to land.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

Looks good to me! It's a shame we have to expose functions just for testing, but the benefits of adding those tests far outweigh the drawbacks.