kiji-express: EXP-292. Modfiy testing framework to return avro records of the correct type (specific or generic).

issue: https://jira.kiji.org/browse/EXP-292
review: https://review.kiji.org/r/1350
commit: https://github.com/kijiproject/kiji-express/commit/fd20683924da66e102a0eca96c05f579ce0a0111