Refactor fairseq/test_noising with a word shuffle helper function (#340)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/340

This allows us to do a lot less copy paste when adding new word shuffle function tests

Reviewed By: xianxl

Differential Revision: D12810304

fbshipit-source-id: a56b5df093d17be2b73837897c526978cab92b70