fix test failure with seed 24594

i18n tests don't seem to stand up to test randomization.  This fixes one case.  Before this patch, if you run the tests like this:

```
$ TESTOPTS='--seed=24594' bundle exec rake test
```

They will fail.  This patch fixes them by testing a subclass of `I18n::Backend::Chain` which lets us avoid mutating the class by including `I18n::Backend::Fallbacks`.