Bug 887777 - Set up the first few tests for keyboard suggest

Fix for [#887777](https://bugzilla.mozilla.org/show_bug.cgi?id=887777). This adds some structure to the keyboard suggestions where we can add some more tests in the future. There are two parts:

1. `latin_suggest_tests`, which covers code that handles how the IM handles response from workers (mocks worker)
2. `worker_tests` which tests the worker and predictions

The extra lines in latin.js are there because we had an undefined global hanging around that we crashed upon in tests.