Tests: the data set export test fails when its folder cannot be emptied (#3389)

variables_panel_data_sets_ask_off_the_ui_thread empties its folder
before Export Data Sets… writes out.csv, then checks that the file is
exactly the header and the row for data set B. It ignored the result
of remove_dir_all, and create_dir_all succeeds on a folder that is
still there. When the folder could not be emptied, an out.csv left by
an earlier run passed the check even when the export wrote nothing.

The test now fails when emptying the folder fails with an error other
than NotFound.

crates/ui-egui/src/tests_picks.rs

@@ -229,7 +229,9 @@ fn variables_panel_data_sets_ask_off_the_ui_thread() {
     let (mut app, shown, _) = app();
     app.run("file.new", json!({"width": 100, "height": 80})).unwrap();
     let dir = vectorcraft_testkit::temp_dir("picks-data-sets");
-    let _ = std::fs::remove_dir_all(&dir);
+    if let Err(e) = std::fs::remove_dir_all(&dir) {
+        assert_eq!(e.kind(), std::io::ErrorKind::NotFound, "cannot empty {}: {e}", dir.display());
+    }
     std::fs::create_dir_all(&dir).unwrap();
     let (csv, out) = (dir.join("sets.csv"), dir.join("out.csv"));
     std::fs::write(&csv, "name,Name\nB,y\n").unwrap();