It depends on your club rules and who you are insured with, and what they advise, for example in the MPBA any boiler (or pressure vessel) not matter what size must have a test certificate. Now if your club follows the Southern Federation (or northern Federation) which is mainly for loco's so testing is a little different, they have the Bar-Litre chart
(also approved by Walker Midgley insurance) It would depend on what size has been approved on the Bar litre chart for not requiring a test.
To calculate: Total water capacity of the boiler (not the working level) x Working pressure, ie:
2 pint boiler (1.137 litres) working at 50 psi (3.447 Bar) =3.919, so if the club (or insurance company) defines that any thing less than 3 bar litres does not require a test, then in this example the boiler would require a test.
My personal opion is test all boilers, then you know you are covered
If you need any more info, let me know