This mod adds simple new item - Damage Tester. You can easily configure (with just an anvil) damage amount you will deal with it as well as different characteristics such as armor piercing, ignoring potions, etc. Attack yourself or other players/mobs. Full guide is also available in the tooltip of the item and as follows:

Rightclick to attack yourself

Damage amount stored in NBT. Sneak-rightclick to read new value from name (floating point/integer number)

Damage characteristics from name (can be combined):
'armor' = Bypasses Armor
'gm1' or 'creative' = Bypasses Creative Mode
'ench' or 'potion' or 'absolute' = Bypasses Potions and Enchantments
'fire' = Fire Damage
'proj' = Projectile Damage
'diff' = Difficulty Scaled (peaceful = 0%, easy = 50%+1, normal = 100%, hard = 150%)
'magic' = Magic Damage
'expl' or 'tnt' = Explosion Damage

Advanced: if you want just to change health by specific amount (via set health)
Rename item to 'set' (incompatible with above types). Negative damage value heals, positive - attacks
This bypasses all damage events and vanilla hurt cooldown

Usage examples:

Rename item to "10" and sneak-rightclick - it will remember damage value of 10.
Rename to "armor" and rightclick. It will cause you 10hp (5 hearts) damage, piercing through any (most of vanilla and modded) armor on you.
Now rename it to "projfire" (or "proj fire" - any separator type and amount works) and attack some other player/mob - it will cause 10 projectile+fire damage (something like fireball), that can be negated by fire resistance potion or reduced by projectile protection enchantment.

Damage Emulator