add checkboxes for unit tests to pr template (#335)

Co-authored-by: user622628252416 <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,9 +9,12 @@
 <!-- Please use Markdown Checkboxes. 
 [ ] = not done
 [x] = done
+(or just click checkboxes to toggle them)
 -->
 ## Checklist
 Things need to be done before this Pull Request can be merged.
 
 - [ ] Code is well-formatted and adheres to project style guidelines: `cargo fmt`
-- [ ] Code does not produce any clippy warnings `cargo clippy`
+- [ ] Code does not produce any clippy warnings: `cargo clippy`
+- [ ] All unit tests pass: `cargo test`
+- [ ] I added new unit tests, so other people don't accidentally break my code by changing other parts of the codebase. [How?](https://doc.rust-lang.org/book/ch11-01-writing-tests.html)