Re-add deleted tests from `prototype/*/non_trivially_relocatable_class/`.

The other tests are pretty uninteresting / already present here, but these three tests were sadly unique to the prototype directory.

(I'd been meaning to delete `prototype/` myself, after moving these tests, but let myself procrastinate too long, oops!)

PiperOrigin-RevId: 585881186
Change-Id: Icbc533988b21034973f6a32b341a67b16ac5b047
diff --git a/support/ctor.rs b/support/ctor.rs
index af3e780..41a6214 100644
--- a/support/ctor.rs
+++ b/support/ctor.rs
@@ -1242,6 +1242,7 @@
 mod test {
     use super::*;
     use std::cell::RefCell;
+    use std::pin::Pin;
     use std::sync::Mutex;
 
     #[test]