The Collaborative International Dictionary of English v.0.48:

Attester \At*test"er\, Attestor \At*test"or\, n.
   One who attests.
   [1913 Webster]

WordNet (r) 3.0 (2006):

attestor
    n 1: (law) a person who attests to the genuineness of a document
         or signature by adding their own signature [syn: witness,
         attestant, attestor, attestator]