The test checks if the captured content does not change.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
{@foo is 5}
<opt:capture as="capt">
FOO: {@foo}
</opt:capture>
{@foo is 6}
{$system.capture.capt}
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
FOO: 5