Tests: the data set export test checks the whole file in a folder emptied first (#2969)

variables_panel_data_sets_ask_off_the_ui_thread checked only that the
exported file started with the header, so an export of the header
alone passed. Its folder was named after the process ID and was used
as it was found, so an out.csv left by an earlier run with the same ID
passed the check even when the export wrote nothing.

The test now empties its folder from vectorcraft_testkit::temp_dir
before use, and checks that the file holds the header and the row for
data set B and nothing else.

crates/ui-egui/src/tests_picks.rs

@@ -222,13 +222,14 @@ fn load_profile_asks_off_the_ui_thread() {
 }
 
 /// The Variables panel menu's Import Data Sets… and Export Data Sets… ask off the UI thread: the
-/// data sets in the file picked are loaded, and the data sets are written to the path the dialog
-/// answers.
+/// data sets in the file picked are loaded, and the header and data set `B` are written to the
+/// path the dialog answers.
 #[test]
 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 = std::env::temp_dir().join(format!("vc-picks-data-sets-{}", std::process::id()));
+    let dir = vectorcraft_testkit::temp_dir("picks-data-sets");
+    let _ = std::fs::remove_dir_all(&dir);
     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();
@@ -244,7 +245,7 @@ fn variables_panel_data_sets_ask_off_the_ui_thread() {
     assert!(matches!(shown.requests().as_slice(), [PickRequest::Save(_)]));
     shown.answer(&[&out.to_string_lossy()]);
     poll(&mut app);
-    assert!(std::fs::read_to_string(&out).is_ok_and(|text| text.starts_with("name,Name\n")), "{}", app.ui.status);
+    assert_eq!(std::fs::read_to_string(&out).ok().as_deref(), Some("name,Name\nB,y\n"), "{}", app.ui.status);
     let _ = std::fs::remove_dir_all(dir);
 }