Fix !Unpin forward declaration test to actually, uh, test forward declarations.

IDK how I messed this one up, it's not even a copy-paste-modify error, just a silly mistake. It was doing a self-cast instead.

PiperOrigin-RevId: 442105693
diff --git a/rs_bindings_from_cc/test/struct/forward_declarations/forward_declarations_test.rs b/rs_bindings_from_cc/test/struct/forward_declarations/forward_declarations_test.rs
index 7d2fd88..3be4601 100644
--- a/rs_bindings_from_cc/test/struct/forward_declarations/forward_declarations_test.rs
+++ b/rs_bindings_from_cc/test/struct/forward_declarations/forward_declarations_test.rs
@@ -22,7 +22,7 @@
     ctor::emplace! {
       let mut s = definition::NonunpinStruct::ctor_new(());
     }
-    let mut incomplete_s: Pin<&mut definition::NonunpinStruct> = s.as_mut().incomplete_cast();
+    let mut incomplete_s: Pin<&mut IncompleteNonunpinStruct> = s.as_mut().incomplete_cast();
     definition::WriteCompleteNonunpinStruct(incomplete_s.as_mut().incomplete_cast(), 42);
     assert_eq!(definition::ReadCompleteNonunpinStruct((&*incomplete_s).incomplete_cast()), 42);
 }