So the problem is that the test runner has to compute the expected results every time she runs the test script.

A slight improvement will be to show the expected results next to the actual values.

examples/php/intro/t02.php
<?php

require_once(dirname(__FILE__) . '/../includes/mylib.php');

print add(1,1)   . ' == 2<br>';
print add(2,2)   . ' == 4<br>';
print add(1,1,1) . ' == 3<br>';

?>

Result:

2 == 2
4 == 4
2 == 3

This is still hard to follow when there are lots of cases or even a few cases with large output but it seems to be a step in the good direction.