Who or what is a Testatrix?
The legal jargon for a person who creates a will is the testator. A female who writes a will used to be called a testatrix.
Used since the 15th century, the trix ending makes a feminine noun for masculine nouns ending in tor. From this rule, we get testator / testatrix, aviator / aviatrix, legislator / legislatrix, and so on. In this enlightened age, the female versions have been dropped from use and the masculine nouns now mean both male and female.
Some legal documents and pedantic people still use the terms testatrix, executrix, and inheritrix when referring to women, but do not let them bother you.