Fix test suite execution from build path

I had a problem running the acceptance test suite in the fitnesse project through the browser. A "git bisect" (what a remarkable tool that is!) marked commit cfcc9b63 as the one introducing the problem.

I made a small fix, introducing a "%P" (capital P) in the command pattern. This marker is replaced by the classpath used to launch FitNesse. This is enough to het stuff working for both the "normal" fitnesse config, as well as the stand alone running.

Downside may be that also plugin jars are added to the classpath.