Explain diff output format

When a test fails you get a unified `diff` output like so:

```
Test 8 failed: output mismatch:
+ . . . .
+ c c c c
+ . . . .
+ . . . .
```

It would be very helpful to explain what this means, or switch to a less confusing output (maybe "This line is different" instead of `-` and `+`.)
