test: respect unstable overlay feature gate

rust/lancedb/src/table.rs

@@ -5763,6 +5763,14 @@ mod tests {
         assert!(index_bytes > 0);
         assert_eq!(with_index, data_only + index_bytes);
 
+        // Release builds reject unstable overlay datasets unless explicitly opted in.
+        if !cfg!(debug_assertions)
+            && std::env::var_os(lance_table::feature_flags::ENABLE_UNSTABLE_DATA_OVERLAY_FILES_ENV)
+                .is_none()
+        {
+            return;
+        }
+
         // Commit an overlay file supplying new `foo` values for the first three
         // rows of fragment 0. There is no high-level API that writes overlays
         // yet, so write the overlay's data file and commit the `DataOverlay`