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.