Yes, ATTESTANT is a Scrabble word!

Letter values in Scrabble US: A 1, T 1, T 1, E 1, S 1, T 1, A 1, N 1, T 1.

Is it a valid word?

ATTESTANT is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

(law) a person who attests to the genuineness of a document or signature by adding their own signature
someone who affirms or vouches for the correctness or truth or genuineness of something

Not playable in Scrabble US: ATTESTATOR.