1. an indication or proof that something exists or is true
a testament/testimony to somebody's perseverance
One of the definitions of the word 'testament,' when taken on its own, is 'to make a statement of belief,' and of 'testimony': 'to provide evidence under oath.' Therefore, when you say that something is a 'testament/testimony to' something, you are providing support for, or giving evidence to prove a claim, opinion, or idea. Both variations of this expression can be used in either a social or professional setting, though 'testament' is seen as being more conventional.