Hi Junus,
the shown test setup has unity gain for dc only - that is necessary to ensure an operating point within the opamp´s linear range. Otherwise, the offset voltage would bring the output into saturation because of the large open-loop dc gain.
The capacitor C should be as large as possible because - together with the large R - it creates a large time constant equivalent to a very low corner frequency wc.
Thus, for frequencies above wc the inverting input (increasingly) is ac grounded and you can measure/simulate the open-loop gain by connecting a swept frequency signal at Vin.
The load componenets (RL and CL) are not too important. Thus, some kind of normal operation conditions are established..
Remark: The rising linear curve starting at 1/RC is the inverse feedback factor 1/Hf with Hf=1/(1+sRC). Thus, the bold line is identical to the gain of the whole circuit (with feedback).
The crossing of both functions gives you the frequency w1=Av(0)/RC above which the measurement is identical to the desired open-loop gain of the opamp alone. This curve clearly shows why you should select a large time constant RC.