tests: Ignore missingType.return PHPStan error until : void types are allowed

Author: justlevine

phpstan.neon.dist

@@ -37,7 +37,9 @@ parameters:
     - '#Default value of the parameter .* is incompatible with type callable.*#'
 
     # Level 6:
-
     # WPCS syntax for iterable types is not supported:
     -
       identifier: missingType.iterableValue
+    # Too noisy until we allow `void` return types:
+    -
+      identifier: missingType.return