test(get.R): add snapshot testing workflow

[droomelotdegendt]

Goal

Introduce a testing setup for the database functions (get_locs, get_xg3, get_chem). While running these tests, a dbplyr query translation error was discovered and subsequently fixed.

Key Changes

Main goal: snapshot testing setup (tests/testthat)

• Implemented an automated testing structure using locally stored CSV snapshots to ensure code stability without exposing confidential data to a public repository.
• Added documentation on how to run these tests for future contributors and reviewers.

Bonus: bugfix discovered during testing

• Fixed a bug in get_chem where modern dbplyr versions generated corrupted SQL expressions by replacing the legacy raw sql() string with native dplyr::between() syntax.

[florisvdh]

Great!!! Thank you very much for providing a consistent testing framework that conforms with testthat & tidyverse guidelines, and for adding awesome documentation! 🚀