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):

attester
    n 1: someone who affirms or vouches for the correctness or truth
         or genuineness of something [syn: attester, attestant]