(maint) Add tests to PR Checklist (#1558)

While the project has only a handful of tests, it would be in the interest of
contributors to at least be prompted to think about adding tests for new
functionality.  This commit updates the PR template and adds a testing checkbox.
Note that these are not mandatory and can even be marked as N/A when people
raise a PR.

Author: glennsarti

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,5 +9,6 @@ Please mark anything not applicable to this PR `NA`.
 
 - [ ] PR has a meaningful title
 - [ ] Summarized changes
+- [ ] PR has tests
 - [ ] This PR is ready to merge and is not work in progress
     - If the PR is work in progress, please add the prefix `WIP:` to the beginning of the title and remove the prefix when the PR is ready