Improve --update-data

[ikonst]

Current --update-data is somewhat crude:

• works by search and replace of substrings
• handles only errors, not warnings or notices
• only changes existing notes, does not add/remove notes
• does not handle multiline notes

While handling all sorts of data suites is a large undertaking, I think we can do a better job for 'check' tests. I have some rudimentary code (that I used privately for a PR that caused a lot of changes) and I wanted to gauge interest on contributing it.

p.s. I agree "snapshots tests", at their worst, can result in cavalier approach to test failures. I hope code reviews can compensate for this. The downside to not having an efficient update mechanism is withholding beneficial refactors due to the cost associated.

[JelleZijlstra]

Sounds like a good idea! PRs that improve error messages often need a lot of tweaks to tests, so it's good to make that easier.