docs: add cargo test requirement to PR checklist
<!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
* [ ] I'm merging against the `development` branch.
* [ ] I ran `cargo-fmt --all` before pushing.
+* [ ] I ran `cargo test` successfully before submitting my PR.
* [ ] I have squashed my commits into a single commit.
* [ ] My change requires a change to the documentation.
* [ ] I have updated the documentation accordingly.
