Don't modify the translation store

Working towards fixing https://www.pivotaltracker.com/story/show/56873538

Modifying the translation store during tests was causing [this test](https://github.com/alphagov/whitehall/blob/e93bca0eb16b2a1c38309033d62bb0408c345680/test/unit/helpers/document_helper_test.rb#L198) to sporadically fail because the translation for "Minister" was being overwritten in the translation store and so it was being translated as 'minister-value'.  I did look into adding a teardown that reset the
translation store, but this is very slow (blowing away and reloading all translations).

Although there is now the risk that these tests will break if the French translations for these words should ever
change, I think this is better than stubbing the translation helper, which feels way too much like testing implementation.