Make highlightCodeBlock render enclosing div tags on separate lines

org/testdata/blocks.html

@@ -1,12 +1,16 @@
 <div class="captioned">
-<div class="highlight"><pre>echo a bash source block</pre></div>
+<div class="highlight">
+<pre>echo a bash source block</pre>
+</div>
 <p class="caption">
 block!
 </p>
 </div>
-<div class="highlight"><pre>a source block without a language
+<div class="highlight">
+<pre>a source block without a language
 and a second line
-and a third one</pre></div>
+and a third one</pre>
+</div>
 <pre class="example">
 an example block
 with multiple lines