test(e2e): wait for theme-directives.html

The `theme-directives.html` test currently sometimes takes a screenshot
before all of the Mermaid diagrams have completed rendering.

We can use the `urlSnapshopTest()` helper function, which waits until
a `.rendered` property exists on the page.

Co-authored-by: Sidharth Vinod <sidharthv96@gmail.com>

cypress/platform/theme-directives.html

@@ -121,7 +121,14 @@
         fontFamily: '"arial", sans-serif',
         curve: 'cardinal',
         securityLevel: 'strict',
+        startOnLoad: false,
       });
+
+      await mermaid.run();
+
+      if (window.Cypress) {
+        window.rendered = true;
+      }
     </script>
   </body>
 </html>