Tests: check where each Export Selection with options writes

The test now asserts that the picked file and the given path receive the two writes, as well as the write count.

crates/ui-egui/src/io.rs

@@ -465,7 +465,10 @@ mod tests {
         assert_eq!(&written.borrow()[0].1[1..4], b"PNG");
         app.run("document.exportSelection", json!({"path": "/tmp/sel2.png", "scale": 2})).unwrap();
         assert_eq!(*asked.borrow(), 1, "a path writes without the save panel");
-        assert_eq!(written.borrow().len(), 2);
+        let w = written.borrow();
+        assert_eq!(w.len(), 2);
+        assert_eq!(w[0].0, "/tmp/sel.png", "the picked file");
+        assert_eq!(w[1].0, "/tmp/sel2.png", "the given path");
     }
 
     #[test]