I don't think it's useful to watch in the first 10 characters of graded result.  You commented in one of the tests about "timeout" being a valid explanation.  Unfortunately that's exactly what happens when there's a timeout:
This is my capture from `tcpdump`
```
{"score":0,"maximum-score":1,"feedback":[{"user_answer":"<result>\n{for $c1 in doc(\"countries.xml\")\/\/country\nlet $d1 := ($c1\/@population div $c1\/@area)\nfor $c2 in doc(\"countries.xml\")\/\/country\nlet $d2:= ($c2\/@population div $c2\/@area)\nwhere $d1 >= $d2\nreturn <highest density = \"{$d1}\">{data($c1\/@name)} <\/highest>}\n\n{for $c1 in doc(\"countries.xml\")\/\/country\nlet $d1 := ($c1\/@population div $c1\/@area)\nfor $c2 in doc(\"countries.xml\")\/\/country\nlet $d2:= ($c2\/@population div $c2\/@area)\nwhere $d1 <= $d2\nreturn <lowest density = \"{$d1}\">{data($c1\/@name)} <\/lowest>}\n<\/result>","score":0,"explanation":"Timeout Error"}]}
```
and if I remember correctly, the reason failures were dastardly on grader102 was because they gave a `{"score":0, "maximum":1, explanation:""} `

In either case, unfortunately, they won't trip this test.