update Scene.toImageSync test to use proper bounds in the request (flutter#37638)

Author: flar

testing/dart/compositing_test.dart

@@ -42,16 +42,16 @@ void main() {
     builder.addTexture(0, width: 10, height: 10);
 
     final Scene scene = builder.build();
-    final Image image = scene.toImageSync(10, 10);
+    final Image image = scene.toImageSync(20, 20);
     scene.dispose();
 
-    expect(image.width, 10);
-    expect(image.height, 10);
+    expect(image.width, 20);
+    expect(image.height, 20);
 
     final ByteData? data = await image.toByteData();
 
     expect(data, isNotNull);
-    expect(data!.lengthInBytes, 10 * 10 * 4);
+    expect(data!.lengthInBytes, 20 * 20 * 4);
     expect(data.buffer.asUint8List()[0], 0);
     expect(data.buffer.asUint8List()[1], 0);
     expect(data.buffer.asUint8List()[2], 0);