All 4 Uses

testament
in
The Wide Window
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As my last will and testament, I leave you three in the care of Captain Sham, a kind and honorable men.†
p. 59.2 *

last will and testament = a final will (written instructions expressing how somebody wants their property distributed after they die)
As my last will and testament, I leave you three in the care of Captain Sham, a kind and honorable men.†
p. 61.6
Someone's last will and testament is an official statement of the wishes of the deceased.†
p. 76.8
Everyone thought you were dead, and you wrote in your will and testament that we should he placed in the care of Captain Sham.†
p. 132.6

will and testament = a will (written instructions expressing how somebody wants their property distributed after they die)

Definitions:

(1) (testament)
evidence or proof of something, or a formal written statement — such as a will or one of the two main parts of the Christian Bible

Most commonly, you can substitute "evidence" or "proof" for testament. But see the first exemplary sample sentences for other common word substitutions.
(2) (meaning too rare to warrant focus)