Added option to run parallel workers in independent working directories

... to allow tests to be parallelized even if they create files on the file system. We are useing parallel-junit on a large, existing code base. Ideally, unit tests shouldn't be creating files in the cwd, but unfortunately, we have quite a few which do. In order to give us time to fix these tests, I added this option. It seems like something that would be useful for other teams with large numbers of tests which do this.
I think I'd prefer to see this default to temp or give an error if the working directory isn't set but per worker is.