kiji-poms: POM-25. run unit tests headless, add -DskipUnitTests flag

issue: https://jira.kiji.org/browse/POM-25
review: https://review.kiji.org/r/1510
commit: https://github.com/kijiproject/kiji-poms/commit/cbb9cf97bdf482cfd002084d75b3f576a4e49606