tests: introduce test_context to handle internationalization

As preparation to the migration to Turbogears2, introduce a test_context
context manager to wrap certain tests. The context manager provides a
translator for tests that need it. This translator was previously provided
globally to all tests via the TestController.

When swapping Pylons with Turbogears2, the new file
kallithea/tests/test_context.py can be removed, and the test_context from
tg2/tg/util/webtest.py should be used.