Tests: two canvas tests render on their own thread

A document's first frame goes to the render worker, and these read the
texture right after it, so under load it had not landed yet.

crates/ui-egui/src/canvas/tests.rs

@@ -2879,6 +2879,7 @@ fn pixel_grid_shows_in_pixel_preview_from_600_percent() {
 #[test]
 fn grid_and_guides_take_their_preferences() {
     let mut app = VectorcraftApp::new(Session::new(), Default::default());
+    app.canvas.worker_started = true; // render on this thread
     app.session.execute("file.new", &json!({"width": 400, "height": 300})).unwrap();
     app.session.execute("guide.add", &json!({"vertical": true, "pos": 100})).unwrap();
     app.ui.view.grid = true;

crates/ui-egui/src/graphics.rs

@@ -195,6 +195,7 @@ mod tests {
     #[test]
     fn the_app_uploads_everything_again_in_a_new_context() {
         let mut app = app();
+        app.canvas.worker_started = true; // render on this thread
         let old = egui::Context::default();
         // Fonts, then the canvas.
         for _ in 0..3 {