Expose use-file option through a project.clj keyword

Makes it possible to write test results to a file instead of stdout. Useful if you plan to integrate the results on a CI server that supports junit xml output.

To use, add this to project.clj

  :junit-formatter :xml
  :junit-formatter-use-file "on"

If :junit-formatter-use-file is set to on, JUnitTask.setPrintSummary will be set to "withOutAndErr" so you still get useful output when running from the commandline.