Add headless UI tests with kittest

src/app.rs

@@ -1246,17 +1246,4 @@ fn welcome_canvas() -> Canvas {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn the_welcome_canvas_is_valid_and_connected() {
-        let canvas = welcome_canvas();
-        assert_eq!(canvas.nodes.len(), 4);
-        assert_eq!(canvas.edges.len(), 3);
-        let mut copy = canvas.clone();
-        assert_eq!(copy.drop_dangling_edges(), 0);
-        // And it round-trips through the file format.
-        assert_eq!(Canvas::from_json(&canvas.to_json()).unwrap(), canvas);
-    }
-}
+mod tests;