Add "test" support to the datasource

[netsniffer]

Hello,

I can get and calculate data point by this plugin ,but I got an error like below when i test the sample data point on 1.0.5 .

Preparing Command...
Executing command $ZENHOME/libexec/ against 10.0.0.1
/bin/sh: /opt/zenoss/libexec/: is a directory
DONE in 0 seconds

CentOS 6.3 / Zenoss 4.2.3

Thank you.

Sheng