Array
(
    [0] => Array
        (
            [p_id] => 1
            [test_id] => 1
            [test_row] => 1
            [answer_1] => +
            [answer_2] => +
            [answer_3] => -
        )

    [1] => Array
        (
            [p_id] => 2
            [test_id] => 1
            [test_row] => 2
            [answer_1] => +
            [answer_2] => +
            [answer_3] => +
        )
     ....

    [570] => Array
        (
            [p_id] => 3
            [test_id] => 1
            [test_row] => 3
            [answer_1] => +
            [answer_2] => +
            [answer_3] => o
        )

{
    test_id_1: {
        row_1: {
            a: +,
            b: o,
            c: -,
        }
        row_2: {
            a: +,
            b: +,
            c: -,
        }
        row_3: {
            a: o,
            b: o,
            c: +,
        }
    },
    test_id_38: {
        row_1: {
            a: -,
            b: -,
            c: +,
        }
        row_2: {
            a: +,
            b: +,
            c: -,
        }
        row_3: {
            a: +,
            b: o,
            c: +,
        }
    }
}

$ids = array_column($data, 'test_id');

$keys = array_map(function($id){ return "test_id_$id";}, array_unique($ids));

$result = array_fill_keys($keys, []);

foreach($data as $v){
    $tkey = "test_id_{$v['test_id']}";
    $rkey = "row_{$v['row_id']}";

    $result[$tkey][$rkey] = [
          'a' => $v['answer_1'],
          'b' => $v['answer_2'],
          'c' => $v['answer_3']
      ];
}