That's not it. Your engine is grounded and the dizz is bolted to the engine, so its grounded by association. Your 12V test probe won't work when both ends are touching energized sources. Both coil terminals pass energy. The negative side of the coil is not a ground. Your probe will only work when one end is touching a chassis ground and the other end is touching an energized source.
Do the following: Remove your lead between the coil and dizz at the dizz end. Expose the brass contact out of the boot. Position the contact so that it is resting near a chassis ground, but with an air gap of @ 3/8ths of an inch. Have a helper crank the starter briefly while you watch for a spark strong enough to jump the air gap to ground. Low light situation will be helpful also. If you do see a good spark, replace that wire onto the dizz cap and pull any spark plug wire boot from the spark plug of your choice. Do the same spark looky test. No need to do this if you don't see spark with the first one.