Specify the path when executing /usr/bin/test

This module doesn't work on my ubuntu 14.04 installation because puppet tries to run `/bin/test`, but the `test` binary is installed to `/usr/bin/test` on all linux systems I have been able to find.

I attempted to run the acceptance tests as per `CONTRIBUTING.md` but I couldn't get them to work at all before my changes, so I have not added an acceptance test for this change.

If you're using this on a system that puts the `test` binary under `/bin` (not sure whether one exists) then `config.pp` will have to be modified to add `/usr/bin` to the `path` parameter instead of fully qualifying the `unless` parameter.