Add tests of QueryTreeDataProvider

[robertbrignull]

This PR adds tests of the QueryTreeDataProvider class.

I investigated if we could make these unit tests rather than integration tests, but sadly it proved too complicated and not worth it for the benefit it gives.

To make this easier, I introduced a new QueryDiscoverer interface which allows us to easily instantiate QueryTreeDataProvider in the tests without dealing with the more complicated QueryDiscovery class and without Jest mocking. Do you think this interface is helpful and worth it? Suggestions on naming welcome?

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.

[shati-patel]

Thanks, tests looks good! 🧪

I don't fully understand the QueryDiscoverer class, but if it makes things easier then that's fine 😅