Update PR template to include checkbox for Unit Test related changes (…

…#627)

.github/pull_request_template.md

@@ -8,6 +8,13 @@ Test Steps
 -----------
 <!-- Describe the steps to reproduce. -->
 
+Checklist:
+----------
+<!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
+<!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
+- [ ] I have tested my changes. No regression in existing tests.
+- [ ] I have modified and/or added unit-tests to cover the code changes in this Pull Request.
+
 Related Issue
 -----------
 <!-- If any, please provide issue ID. -->