Fix default HighlightCodeBlock fn: Write tags on separate lines

org/testdata/blocks.html

@@ -1,19 +1,23 @@
 <div class="captioned">
 <div class="highlight">
-<pre>echo &#34;a bash source block&#34;
+<pre>
+echo &#34;a bash source block&#34;
 
 function hello {
     echo Hello World!
 }
 
-hello</pre>
+hello
+</pre>
 </div>
 <p class="caption">
 block caption
 </p>
 </div>
 <div class="highlight">
-<pre>a source block without a language</pre>
+<pre>
+a source block without a language
+</pre>
 </div>
 <pre class="example">
 an example block with