Explanation:
A test charge certainly does affect the electrical field.
But if you’re talking about the concept of Potentials, then read carefully: they’re talking about imaginary test-charges which have infinitesimal charge.
Any real test-charge will have a significant effect on the e-field being tested.
However, if we keep making our test-charge smaller and smaller, the effect will grow less and less. Therefore just make our test-charge almost zero, but not exactly zero. Let its charge become infinitely close to zero. Then it will have an infinitesimal e-field of its own, which doesn’t alter the e-fields being measured.
