Interested to know the procedure used to test at different pressures. I believe the service manual test is to ensure spark at least to 7mm gap. (far more than the .020-.024" plug gap) 

Alnarv,  many decades ago a pressurized spark plug tester was apparently not uncommon.  The antique one I saw used a Model T ignition coil and had a pressure chamber with a thick quartz(?) glass window.  You'd thread the plug into the chamber and pump up the chamber with compressed air while viewing the spark through the window.   To test the ignition system, you'd generate the spark from the engine instead of the Model T coil mounted on the device.  It was probably from the 1920's or 30's?  You would have needed a thread adapter for modern plugs as the base of the spark plug it was designed for was much bigger than modern plugs.  

Starting in the 1950's or 60's a scope could be used to tell you if the plug is firing under compression in the engine.  I have an antique "Champion Spark Plug Tester" that you connect to the battery and plug and then observe the trace through a little round window.  One of the old time mechanics where my Dad worked showed me how to use it to tell the spark plug in my mini bike was bad when I was about 10.  When the mechanic retired a couple of years later, he gave the Champion scope to my Dad to give to me (along with an old Simpson Multimeter that I still use.)