Yes, ATTESTING is a Scrabble word!

Letter values in Scrabble US: A 1, T 1, T 1, E 1, S 1, T 1, I 1, N 1, G 2.

Is it a valid word?

ATTESTING is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

provide evidence for; stand as proof of; show by one's behavior, attitude, or external attributes
authenticate, affirm to be true, genuine, or correct, as in an official capacity
give testimony in a court of law
establish or verify the usage of

Also: take the stand, bear witness.