Update dev docs

Author: MartinBraquet

docs/development.md

@@ -19,4 +19,17 @@ Then update the database:
 
 ```bash
 npx prisma migrate dev --name add-some-profile-variable
-```
+```
+
+### Cover with tests
+
+Best Practices
+
+* Test Behavior, Not Implementation. Don’t test internal state or function calls unless you’re testing utilities or very critical behavior.
+* Use msw to Mock APIs. Don't manually mock fetch—use msw to simulate realistic behavior, including network delays and errors.
+* Don’t Overuse Snapshots. Snapshots are fragile and often meaningless unless used sparingly (e.g., for JSON response schemas).
+* Prefer userEvent Over fireEvent. It simulates real user interactions more accurately.
+* Avoid Testing Next.js Internals . You don’t need to test getStaticProps, getServerSideProps themselves—test what they render.
+* Use jest.spyOn() for Internal Utilities . Avoid reaching into modules you don’t own.
+* Don't test just for coverage. Test to prevent regressions, document intent, and handle edge cases.
+* Don't write end-to-end tests for features that change frequently unless absolutely necessary.