Mermaid: Add data-testid

Author: nginx-jack

layouts/_default/_markup/render-codeblock-mermaid.html

@@ -1,5 +1,5 @@
 {{- $grid := .Attributes.grid | default "first-two-thirds" -}}
-<pre class="mermaid" data-grid="{{ $grid }}">
+<pre class="mermaid" data-grid="{{ $grid }}" data-testid="codeblock-mermaid">
    {{- .Inner | safeHTML }}
  </pre>
  {{ .Page.Store.Set "hasMermaid" true }}