Mower repairers sometimes have a test-board for checking ignition units and coils. This can be done in various ways, most of them electronic. If you wanted to test it yourself you'd probably need to set up a flywheel the same diameter as your chainsaw's flywheel, on the shaft of an electric motor, and mount the coil close to the flywheel. If you decide to try that, make a proper, strong rig with a guard around it: you don't want bits to come loose or fly off. Remember, you need to use a flywheel with a magnet embedded in it, or there won't be any spark regardless of how good your test rig is. A single phase induction motor from a smallish electric fan would probably have a maximum speed of 3,000 rpm. I know your chainsaw probably reaches about 14,000 rpm, but it just isn't safe to run a test at that speed on a home-made rig, in my opinion.
