The test jig must not interfere with the UUT as little as possible by reducing EMI. The wires and UUT must have matched impedance. Possibly shield the wires to reduce EMI, have decoupling capacitors in the Jig. The goal of the jig is to affect as little as possible in the UUT while testing its functionality.
Ideally you would be able to test the UUT as if the jig was not there at all.
Other than not interfering with the digital circuit, the Jig must be capable of picking up high speed oscillations to ensure accurate measurement of the UUT.