github: add reminder about adding tests
Thanks, GitHub Copilot, for adding this item to the checklist :)
@@ -13,3 +13,4 @@ https://github.com/martinvonz/jj/blob/main/docs/contributing.md for details.
If applicable:
- [ ] I have updated `CHANGELOG.md`
- [ ] I have updated the documentation (README.md, docs/, demos/)
+- [ ] I have added tests to cover my changes
