$arResult['SECTIONS'] = [
    ['NAME' => 'test1', 'DEPTH_LEVEL' => 1, 'LEFT_MARGIN' => 1, 'RIGHT_MARGIN' => 14],
    ['NAME' => 'test2', 'DEPTH_LEVEL' => 2, 'LEFT_MARGIN' => 2, 'RIGHT_MARGIN' => 5],
    ['NAME' => 'test3', 'DEPTH_LEVEL' => 2, 'LEFT_MARGIN' => 6, 'RIGHT_MARGIN' => 13],
    ['NAME' => 'test4', 'DEPTH_LEVEL' => 3, 'LEFT_MARGIN' => 7, 'RIGHT_MARGIN' => 12],
    ['NAME' => 'test5', 'DEPTH_LEVEL' => 3, 'LEFT_MARGIN' => 8, 'RIGHT_MARGIN' => 11],
    ['NAME' => 'test6', 'DEPTH_LEVEL' => 3, 'LEFT_MARGIN' => 9, 'RIGHT_MARGIN' => 10],
    ['NAME' => 'test7', 'DEPTH_LEVEL' => 2, 'LEFT_MARGIN' => 14, 'RIGHT_MARGIN' => 15],
];

function buildTree($items, $left = 1, $right = PHP_INT_MAX, $depth = 1): array
{
    $outputArray = [];
    foreach ($items as $item) {
        if ($item['LEFT_MARGIN'] > $left && $item['RIGHT_MARGIN'] < $right && $item['DEPTH_LEVEL'] == $depth) {
            $item['ITEMS'] = buildTree($items, $item['LEFT_MARGIN'], $item['RIGHT_MARGIN'], $item['DEPTH_LEVEL'] + 1);
            $outputArray[] = $item;
        }
    }
    return $outputArray;
}

$arResult['SECTIONS'] = buildTree($arResult['SECTIONS']);