How do I summon a mob captain to test it?

You can run execute as @e[type=<MOB_TYPE>,limit=1,sort=nearest] run function ps-mob:convert/<RARITY> to convert the nearest mob of said type to a captain of said rarity.

So if you run execute as @e[​type=zombie,limit=1,sort=nearest] run function ps-mob:convert/legendary that should convert the nearest zombie to a legendary one.