Add ability to await the discovery refresh promise

[robertbrignull]

The intent of this PR is to give you access to a promise for the Discovery class refresh. This allows us to make the tests more type safe and less prone to flakiness.

One part of this PR is making refresh itself return a promise. This means in the tests we can do await discovery.refresh() instead of having to do await (discovery as any).discover() and calling the private discover method. This means we're no longer circumventing the public interface of the Discover class.

Another part of this PR is adding a getCurrentRefreshPromise method which means even if we weren't the ones to kick off the refresh directly we can still safely wait for it to be finished. This is useful when we are testing listeners to events that trigger a refresh, such as the listener for files changing on disk. We can now do await discovery.getCurrentRefreshPromise() instead of having to do await sleep(100) and hope for the best.

The commits are split up into separate changes, if you prefer to review commit-by-commit.

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! I've read through the commits and I think I understand the idea 😅 As far as I could tell (from local testing), the behaviour is still the same too 🎉

I'm not super confident on the actual code changes, so I wouldn't mind a second look from a more confident TypeScript-y person 👀 ❤️

[robertbrignull]

I think all comments are addressed now, and the tests are passing again. Should be ready for a quick re-review.

[koesie10]

LGTM, thanks for implementing the suggestions