This test queries `extOutput` for the first line of output of the
command which was executed when snmpd started (`HexTest` in snmpd.conf).
Since there is no `/bin/sh` on Windows, no command would be run, and as
such we received an empty string. We fix that by dynamically adjusting
snmpd.conf to run a JScript which has the same output as bigtest.
We also make the test diff more helpful in case of failures, where so
far we only would have known that the output had a different message
digest.
