Add error reminders to array_debug test

Author: chitoyuu

gdnative-core/src/core_types/variant_array.rs

@@ -616,6 +616,9 @@ godot_test!(
     test_array_debug {
         use std::panic::catch_unwind;
 
+        println!("  -- expected 4 'Index 3 out of bounds (len 3)' error messages for edge cases");
+        println!("  -- the test is successful when and only when these errors are shown");
+
         let arr = VariantArray::new(); // []
         arr.push(&Variant::new("hello world"));
         arr.push(&Variant::new(true));