test(gpu): match the planning error instead of err().expect()

crates/gpu/src/plan.rs

@@ -1899,7 +1899,7 @@ mod tests {
         let mut doc = Document::new("nest", Size::new(8, 8), ColorMode::Rgb, SampleType::U8);
         doc.layers = layers;
         let started = std::time::Instant::now();
-        let error = plan(&doc).err().expect("2^30 plannings must be refused");
+        let Err(error) = plan(&doc) else { panic!("2^30 plannings must be refused") };
         assert!(error.0.contains("too many nested layers"), "{}", error.0);
         assert!(started.elapsed().as_secs() < 10, "{:?}", started.elapsed());
     }