Surefire reporter tests use tmp dir located in target/

Currently the Surefire reporter tests are storing the data directly in the repo root/workspace. I know the "workspace" dir is ignored in .gitignore, but he directory is still there and it's IMO a bit confusing. Moving it under the target/ makes it clearer that the directory is a product of build/testing and can be safely removed in case needed (and will be automatically removed during "clean" phase).