chore: mention negative tests in PR template (tact-lang#309)

Co-authored-by: Novus Nota <[email protected]>

.github/pull_request_template.md

@@ -6,6 +6,6 @@ If your PR doesn't close a particular issue, please, create the issue first and
 -->
 
 - [ ] I have updated CHANGELOG.md
-- [ ] I have added unit tests to demonstrate the contribution is correctly implemented
+- [ ] I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
 - [ ] I have run all the tests locally and no test failure was reported
 - [ ] I did not do unrelated and/or undiscussed refactorings