The problem with "It's in the comments" or "Read the source code if you want to know what's going on" is that it puts the burden of effort onto whoever's looking into the problem, where an error message that says what went wrong gives them a huge head-start.
Also, if the output looks like <code>
1. pass
2. pass
3. pass
02_classes/02_functions/031.cf FAIL
</code>
it's very non-obvious to the reader what went wrong - all the subtests (mentioned) passed, right ? - because the subtests that failed weren't mentioned.  So reversing the tests and reporting the subtests that fail would be clearer for whoever's going to look into the failure when someone breaks this.  (It's also better to produce minimal output on passing tests, generally.)