update the code block page, add tests

docs/src/demos/Extensions/CodeBlock/index.vue

@@ -36,8 +36,19 @@ export default {
         CodeBlock(),
       ],
       content: `
-        <p>This is a code block:</p>
-        <pre><code>const foo = 'bar'</code></pre>
+        <p>That’s a boring paragraph followed by a fenced code block:</p>
+        <pre><code>for (var i=1; i <= 20; i++)
+{
+  if (i % 15 == 0)
+    console.log("FizzBuzz");
+  else if (i % 3 == 0)
+    console.log("Fizz");
+  else if (i % 5 == 0)
+    console.log("Buzz");
+  else
+    console.log(i);
+}</code></pre>
+        <p>Press Command/Ctrl + Enter to leave the fenced code block and continue typing in boring paragraphs.
       `,
     })