There are several (#18183, #19430, #18111) tickets that touch either a script or how dhclient calls the script.

It would be useful if these were tested, especially when changes are introduced. Tomek started working on #19430, but the work quickly outgrown its initial scope. After jabber chat between Thomas and Tomek, the decision has been made to move the test framework to a separate ticket. 
This ticket is the result of that discussion.