kiji-mapreduce: KIJIMR-277. Cleanup leaked KijiTable instance in test

issue: https://jira.kiji.org/browse/KIJIMR-277
review: https://review.kiji.org/r/1836
commit: https://github.com/kijiproject/kiji-mapreduce/commit/6580eaf1ef79f76354cb116000e7daad374908dd