Source : Webster's Revised Unabridged Dictionary (1913)

Attester \At*test"er\, Attestor \At*test"or\, n.
   One who attests.

Source : WordNet®

attestor
     n : (law) a person who attests to the genuineness of a document
         or signature by adding their own signature [syn: {witness},
          {attestant}, {attestator}]