build: update grumphp config

Signed-off-by: Bart Jaskulski <bartek.jaskulski@wpdesk.net>

grumphp.yml

@@ -3,3 +3,6 @@ grumphp:
         phpcs: ~
         phpstan:
           memory_limit: "-1"
+          configuration: phpstan.neon.dist
+          ignore_patterns:
+            - tests