Update the test plan and add non-MRVA cases

[robertbrignull]

Going through the test plan I noticed a few places with outdated information or steps.

We have also been considering adding non-MRVA test cases to this because currently it is a bit too MRVA-focussed. I've added cases for downloading databases and running a few types of local queries.

I realise this just makes the test plan longer. We could possibly remove the cases for "problem but not path-problem" queries, since they are largely covered by the path-problem case, but otherwise I don't think we can remove much without lower our coverage quite a bit.

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.