Regarding testing the coil, i'm slightly guessing here, but afaik, the black boxes only replace the points and condenser.. So, otherwise the magneto acts like any other ignition.
If so, the spinning magneto will still generate and deliver energy to the ignition coil's primary winding. And a battery would not be needed to test the coil.
Set a volt meter to it's highest range, connect the meter's + lead to the coil's negative connection, and then ground the meter's negative lead. Kick the engine over and a high voltage should show on the meter.
(It would be at this point that the black box would take over, chop this primary voltage and thus time delivery of the spark to the plug).
If the meter shows voltage it says that the magneto and ignition coil are ok ..
(BUT, i admit i'm not sure about it.. There could be more to it and this test might be inconclusive. No primary coil voltage might be due to trouble with the electronics in the box.)