Code
cat(test_lines, sep = "\n")
Output
<div id="column-1" class="section level2">
<h2>Column 1</h2>
<div id="a-wide-plot" class="section level3">
<h3>A wide plot</h3>
<div class="knitr-options" data-fig-width="960" data-fig-height="480">
</div>
<p><img src="data:image/png;base64,..." width="960" data-figure-id="fig1" /></p>
</div>
<div id="a-tall-plot" class="section level3">
<h3>A tall plot</h3>
<div class="knitr-options" data-fig-width="480" data-fig-height="960">
</div>
<p><img src="data:image/png;base64,..." width="480" data-figure-id="fig2" /></p>
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