Test dropped cell counting from command_process_relay_cell()
It would be ideal to test the dropped cell account all the way from command_process_relay_cell(). To do this, we need to set up fake channels, circuitmuxes, and circuits with fully valid cpaths, which is a fair amount of work.
As far as I know, nothing tests cell processing all the way through command_process_relay_cell(). So this might be useful for other tests, too.