ui-egui tests: hold OK's evaluation where a test checks it is computing (#4040)

Two OK-off-the-window-thread tests checked that OK was computing right
after Enter without holding the evaluation. The preview's evaluation is in
the cache, so the worker was usually in before OK's zero wait was over and
OK applied at once: cancel_while_computing_applies_nothing failed in most
runs (5 of 5 in a debug build, 4 of 5 with the ci profile). Both now hold
the evaluation with the test gate, as their neighbour does, and let it go
once the computing state is checked.

crates/ui-egui/src/apply_off_thread_tests.rs

@@ -64,11 +64,16 @@ fn ok_computes_off_the_window_thread_saying_so_then_applies() {
 fn cancel_while_computing_applies_nothing() {
     let mut h = square_on_a_block();
     h.app.apply_off_thread = Some(Duration::ZERO);
+    // Held, as the preview's evaluation is in the cache and would otherwise be in before OK's
+    // wait is over, applying at once.
+    let (finish, gate) = std::sync::mpsc::channel();
+    h.app.apply_gate = Some(std::sync::Arc::new(std::sync::Mutex::new(gate)));
     let features = h.app.session.doc.features.len();
     h.step(&json!({"key": "Enter"})).unwrap();
     assert!(h.app.applying.is_some());
     h.step(&json!({"key": "Escape"})).unwrap();
     assert!(h.app.applying.is_none() && h.app.dialog.is_none());
+    finish.send(()).unwrap();
     h.frames(5);
     assert_eq!(h.app.session.doc.features.len(), features);
 }
@@ -81,9 +86,13 @@ fn cancel_while_computing_applies_nothing() {
 fn ok_computing_off_the_window_thread_does_not_apply_in_the_frame_it_starts() {
     let mut h = square_on_a_block();
     h.app.apply_off_thread = Some(Duration::ZERO);
+    // Held past OK's wait (the evaluation is in the cache), then let go before the next frame.
+    let (finish, gate) = std::sync::mpsc::channel();
+    h.app.apply_gate = Some(std::sync::Arc::new(std::sync::Mutex::new(gate)));
     let features = h.app.session.doc.features.len();
     h.step(&json!({"key": "Enter"})).unwrap();
     assert!(h.app.applying.is_some(), "the dialog is computing");
+    finish.send(()).unwrap();
     assert_eq!(h.app.session.doc.features.len(), features, "nothing applied in the frame it started");
     // A frame later the evaluation is in and the feature is applied.
     until_applied(&mut h);