github: add 'testing' to the PR template

Add a section on the testing procedure for a pull request.
It should help reviewing pull requests as they often not describe which
application raises the error they are fixing or any way of verifying
that a feature is correctly implemented.

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,6 +15,16 @@ Put here the description of your contribution:
 -->
 
 
+### Testing procedure
+
+<!--
+Details steps to test your contribution:
+- which test/example to compile for which board and is there a 'test' command
+- how to know that it was not working/available in master
+- the expected success test output
+-->
+
+
 ### Issues/PRs references
 
 <!--