Add new Testing Instructions section to PR template. (#38147)

* Add testing instructions to PR template.

* Update phrasing.

* Reviewer feedback.

* Reviewer feedback.

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,10 +5,11 @@
 ## Description
 <!-- Please describe what you have changed or added -->
 
-## How has this been tested?
-<!-- Please describe in detail how you tested your changes. -->
-<!-- Include details of your testing environment, tests ran to see how -->
-<!-- your change affects other areas of the code, etc. -->
+## Testing Instructions
+<!-- Please include step by step instructions on how to test this PR. -->
+<!-- 1. Open a Post or Page. -->
+<!-- 2. Insert a Heading Block. -->
+<!-- 3. etc. -->
 
 ## Screenshots <!-- if applicable -->