test(e2e): wait for marker_unique_id.html e2e test

Sometimes, Cypress takes the screenshot for marker_unique_id.html early,
before mermaid has finished rendering the diagrams.

Fixes: 924c9e9

cypress/integration/rendering/marker_unique_id.spec.js

@@ -2,9 +2,16 @@ import { urlSnapshotTest } from '../../helpers/util.ts';
 
 describe('Marker Unique IDs Per Diagram', () => {
   it('should render a blue arrow tip in second digram', () => {
-    urlSnapshotTest('http://localhost:9000/marker_unique_id.html', {
-      logLevel: 1,
-      flowchart: { htmlLabels: false },
-    });
+    urlSnapshotTest(
+      'http://localhost:9000/marker_unique_id.html',
+      {
+        logLevel: 1,
+        flowchart: { htmlLabels: false },
+      },
+      undefined,
+      ($svg) => {
+        expect($svg).to.have.length(4);
+      }
+    );
   });
 });