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

org/testdata/captions.html

@@ -2,7 +2,9 @@
 Anything can be captioned. Also captions are not real, correct captions but just a paragraph below the element (bothe wrapped into a div)
 </p>
 <div class="captioned">
-<div class="highlight"><pre>echo &#34;i have a caption!&#34;</pre></div>
+<div class="highlight">
+<pre>echo &#34;i have a caption!&#34;</pre>
+</div>
 <p class="caption">
 captioned soure block
 </p>

org/testdata/footnotes.html

@@ -40,7 +40,9 @@ and other elements
 <p>
 like blocks
 </p>
-<div class="highlight"><pre>other non-plain</pre></div>
+<div class="highlight">
+<pre>other non-plain</pre>
+</div>
 </li>
 <li>
 <p>

org/testdata/lists.html

@@ -48,7 +48,9 @@ and some lines of text
 <p>
 and another subitem
 </p>
-<div class="highlight"><pre>echo with a block</pre></div>
+<div class="highlight">
+<pre>echo with a block</pre>
+</div>
 </li>
 <li>
 <p>