Is there any way that I can use the testfor command to constantly fill up the hunger bar? Is so, please leave the command below.
The best and easiest way to do this would be to give all players the saturation effect constantly, instead of trying to do it only when a player is hungry.
/effect @a 23 1000000 1 1
Which gives all players Saturation I for 1000000 seconds, and hides the particles given off by the effect. The advantage to setting the time to such a large number is so that the effect remaining time displays as
xx:xx to indicate to the player that the saturation effect is permanent, and that they don't need to worry about food.
Run this command on a clock input of some sort and hide command block output with
/gamerule commandBlockOutput false and you're all set.