Improving CI mutation check

We only want to test for mutations on covered code (and ignoring Interfaces as they provide no value in mutation testing)

.circleci/config.yml

@@ -77,4 +77,4 @@ jobs:
 
       - run:
           name: Execute mutation testing
-          command: ./vendor/bin/infection --configuration=infection.json.dist --coverage=build/logs/coverage
+          command: ./vendor/bin/infection --configuration=infection.json.dist --coverage=build/logs/coverage --only-covered