[A GFCI receptacle, new or old, can’t trip with a plug in tester unless there is an EGC connected to the GFCI. There is no path for the test current to flow on without an EGC. When testing GFCIs with the internal test button, you must test for voltage on the GFCI receptacle after you push the test button. A GFCI receptacle that has the power connected to the load terminals will still have power( on older models) on the receptacle even when the button shows that the device has tripped.
The GFCI works because it detects the difference between the grounded and ungrounded conductor.
GFCI is a Ground Fault Circuit Interrupter. The Ground Fault Interrupter is a receptacle that has the ability to open or disconnect the power from the output of the receptacle. The Ground Fault Circuit Interrupter is a breaker that has the ability to disconnect the power from a circuit. The ground fault senses a difference in the flow of current from the hot wire through the neutral, if that difference is about 5 milliamps or more the ground fault will trip the circuit out. It actually assumes that if the current is not flowing in the neutral it is flowing through something else. Some motor windings have sufficient losses to cause one to trip out so don’t use a GFCI circuit for a refrigerator or washer outlet. You should use (and the NEC requires) the use of GFCI protected outlets within 6 feet of a sink, anywhere in a bathroom, in a garage or outside; anywhere an outlet can be reached from a water source, a wet area, or earth ground, you should use GFCI protection. A GFCI receptacle has a line side (incoming power) and a load side (outgoing power). The receptacle will not work if the incoming power is connected to the load side of the receptacle. Connect the incoming power to the line marked terminals and the continuation of the circuit (the next outlet) to the load terminals. The one GFCI will protect all the following plugs or receptacles connected in this way. Even if you don’t have a continuation of the circuit, connect the power to the line side of the receptacle. GFCI receptacles and GFCI breakers have a test button that should cause the circuit to trip, operate the test button after installing and regularly there after to be sure it works properly.]