actually, a will is a kind of testament - not vice versa:
tes·ta·ment (tes't?-m?nt)
n.
- Something that serves as tangible proof or evidence: The spacious plan of the city is a testament to the foresight of its founders.
- A statement of belief; a credo: my political testament.
- Law. A written document providing for the disposition of a person's property after death; a will.
- Testament Bible. Either of the two main divisions of the Bible.
- Archaic. A covenant between humans and God.