Add note about possibly flaky test

This assertion failed in CI GNUStep 32bit with the retain count being 2 instead of the expected 1. If I see this issue again, I'll try to investigate it

Author: madsmtm

objc2/src/rc/id.rs

@@ -499,6 +499,7 @@ mod tests {
         });
 
         // make sure that the autoreleased value has been released
+        // TODO: Investigate if this is flaky on GNUStep
         assert_eq!(retain_count(&*cloned), 1);
     }