Unit testing requirement added (#1440)

* Unit testing requirement added
* Wording updated
* Headline updated

Author: olganaumenko

.github/pull_request_template.md

@@ -16,6 +16,10 @@ Please delete options that are not relevant.
 
 # How Has This Been Tested?
 
+## Regression and integration tests
+
+Please, provide regression or integration tests for your code changes. If you don't do that, the reviewer can and highly likely **_will reject_** the PR. It is the contributor's responsibility to provide such tests or to reason why they are missing.
+
 ## Automated Testing
 
 Specify tests that help to verify the change automatically.