docs: add testing steps to PR template

.github/pull_request_template.md

@@ -1,9 +1,16 @@
 ## Describe your changes
 
+<!--
+Describe what's changed and why. If interactive testing is required, explain
+to the reviewer how the PR should be tested.
+-->
+
 ## Issue ticket number and link
 
 ## Checklist before requesting a review
 
+- [ ] I have added guiding text to explain how a reviewer should test these changes.
+
 - [ ] If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:
 
   > REPLACE THIS TEXT WITH RATIONALE (CAN BE BRIEF)