Update test confirmation text in PR template (#2897)

* Update test confirmation text in PR template

We only ask users to check when adding new tests, but
some PRs only modify existing tests. Also added
instructions to mark with an "x" for checking off items.

Author: superkhau

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,6 +14,11 @@ Please use the following link syntaxes:
 
 ### Checklist
 
-- [ ] New tests are added to cover all changes
+<!--
+Please mark your choice with an "x" (i.e. [x], see
+https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments)
+-->
+
+- [ ] New tests added or existing tests modified to cover all changes
 - [ ] Code conforms with the [style
   guide](http://loopback.io/doc/en/contrib/style-guide.html)