Add support for running in a build.gradle that is run from a GradleBuildTask (i.e. subproject)

The change is to pass the project directory to the commandRunner when calling xcodeBuild.
This means that the xcode task can be in a build.gradle that is in a project subfolder (along with the xoce project files), and can be called from a master build.gradle

Also added support for running the plugin test from a windows box (not assuming '/' as the file separator)