Tweaks for integration tests

[Jelenkee]

What's up?

Some follow-ups for #2286

• Remove previous integration tests, move the data files into this path
• Run in-process DBs even without an external connection (will be quite helpful if we start using this more)
• Add a description & link to https://github.com/prql/prql/blob/7ae804a9d3f42f6ed3d33c04ab910dca164c7efc/web/book/src/contributing/development.md#L310
• Implement whether to run external integration tests with a feature, rather than reading the CI env var (that could become a bit confusing otherwise)

• (minor) Possibly output the snapshots as CSVs / something both machine & person-readable, e.g. with headers, rather than one long line
• (minor) Consider whether we keep it in the current CI test vs move to another, possibly in test-all.yaml. It currently adds 40 seconds, which is not too bad. Or we condition the test-rust tests on a PR's paths, and avoid running it for small doc changes.
• When starting docker compose the owner of prql-compiler/tests/integration/data/chinook is changed on Linux. Probably on macOS too. That should be fixed because you cannot edit the files in there. chore(test): bind mount read only in compose file for testing #2352

(edited by @max-sixty to add an item that @Jelenkee suggested. And now completed)

[max-sixty]

Implement whether to run external integration tests with a feature, rather than reading the CI env var

Specifically for this, I was thinking:

• We have a feature in prql-complier called something like test-external-dbs, which is not default
• We put the list of tests to run under the feature (or use cfg-if if needed), so it's the longer list when that feature is active
• Run in CI with --all-features
• Remove the special-case code around CI from the library
• (minor) Also put dependencies behind this feature, so we didn't need to compile them otherwise

This would mean we get dependable, hermetic external tests, and still get the in-process tests by default