Use messages in tests/i18n for tests

'setup.py test' fails calling twtranslate as 'scripts' is not packaged.

Change-Id: I3cb4d071b4dce921e74e91af7b5b613a117e90b4
Original-Change-Id: Ie4f77a8ace594619fee98996cc4403915316b51b
Merge "Use messages in tests/i18n for tests"