Port testrun compat_addons from hotfix-1.5 to master (#69)

Pull request for review.

I did a testrun_compat_addons with the 24.0esr build and the default json.example config, and it appears to be running correctly.

Tested in:
Windows XP32 SP3
mozmill-env 1.5.24.1 latest + mozmill master latest + this branch

Run result:
http://mozmill-addons.blargon7.com/db/456bebe92845279408c15c03e874eb81

Adding @whimboo and @davehunt for visibility.