kiji-model-repository: KIJIREPO-54. Clean up resource leaks in tests

issue: https://jira.kiji.org/browse/KIJIREPO-54
review: https://review.kiji.org/r/1844
commit: https://github.com/kijiproject/kiji-model-repository/commit/44ffaaaf094b45b8411585d93d0c04e75f0606c7