Tests the opt:capture attribute.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
<foo opt:capture="capt">Bar</foo>
BAR
{u:$system.capture.capt}
{u:$system.capture.capt}
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
BAR
<foo>Bar</foo>
<foo>Bar</foo>