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Start a dbus-daemon instance and set DBUS_SESSION_BUS_ADDRESS. After this call, it is safe for unit tests to start sending messages on the session bus.
If this function is called from setup callback of g_test_add(), g_test_dbus_down() must be called in its teardown callback.
If this function is called from unit test's main(), then g_test_dbus_down() must be called after g_test_run().