Test charge
Also called Probe charge
A test charge is a point charge small enough in magnitude that its own presence does not significantly affect the electric field it is being used to measure. That smallness is what makes E equals F over q a definition of the field rather than a rearrangement.
EK 10.3.A.2.i in AP Physics 2, and the identical EK 8.3.A.2.i in C: E&M, put it exactly: a test charge is a point charge of small enough magnitude such that its presence does not significantly affect an electric field in its vicinity.
Two different idealisations. A point charge is small in size, so it has a definite location. A test charge is also small in effect, so it does not disturb what it measures. Every test charge is a point charge; the reverse is not true.
Why the second condition matters. The field statement is
Bring a large probe up to a charged conductor and its own field pushes the source charges around on that conductor's surface. The force you then measure, divided by your probe charge, is not the field that was there before you arrived. Shrink until that back-reaction is negligible and the ratio stops depending on the probe at all, which is the only reason the ratio can be called a property of the point.
Sign convention. Take the test charge positive. EK 10.3.A.2.iii says the force on a positive test charge is in the same direction as the field, so a negative probe would flip the answer.
The same device defines potential: EK 10.5.A.3 describes potential difference as the change in electric potential energy per unit charge when a test charge is moved between two points.