The threaded tests are using the default main context in the worker
thread, but were not g_main_context_acquire()ing it first, which meant
that g_tls_interaction_invoke_ask_password() in the main thread would
sometimes succeed in acquiring it itself and thus performing the
operation in the wrong thread. Fix that.

Also, we can't unref the loop from the worker thread, because the main
thread isn't holding a reference on it, and so it might end up being
destroyed while that thread is still inside g_main_loop_quit().