Engine tests: drop the drop() of a closure (clippy drop_non_drop)

Clippy 1.97 rejects `drop(write)` in the same-folder export test: the
closure has no Drop, so its borrow of `written` already ends at its last
use and the call did nothing. Removed, with a note on why the following
read of `written` compiles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J2NDVXA8jQQWeJb4YVTb6b

crates/engine/src/tests_export.rs

@@ -1038,7 +1038,7 @@ fn same_folder_as_original_refuses_photos_without_a_folder() {
         assert_eq!(f["photo"], id.0, "{f}");
         assert!(f["error"].as_str().unwrap().contains("no folder on disk"), "{f}");
     }
-    drop(write);
+    // (`write` borrows `written` until its last use above; a closure has no `Drop` to wait for)
     assert!(written.is_empty(), "{written:?}");
 
     // a photo on disk whose folder is gone (moved, an unplugged drive)