The Collaborative International Dictionary of English v.0.48:

Attestant \At*tes"tant\ n.
   1. (Law) a person who attests to the genuineness of a
      document or signature by adding their own signature.

   Syn: witness.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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