I will concede to Slomo on this one
A globe type spark tester has a very low resistance so will glow at a very low voltage
So if the coil is only putting out 500 V then you will get a flash from a globe type
However to jump the spark plug gap you need 30,000 V
So if you get a flash from the tester when directly shorted to the block but not when it is connected to a spark plug then you know the trigger unit is working ( points in your case ) but the coil output is too low so the windings are faulty or the HT lead has too much resistance
IF you are using a resistor plug it will usually not work properly in a points ignition.