Add necessary paths to tests to fix bug 2814 (second try)

resubmit of [PR#34](https://github.com/translate/translate/pull/34) - there's no way to rewrite history for a branch once pushed to GitHub, and PRs are permanently tied to a given branch, so to submit with clean history a new PR is needed

In addition to the tests at top level, there's a reference to pocompile in translate/storage/test_mo.py that needs to get paths set so that it can use tools that are part of other parts of translate toolkit