Unity gain isn't a topology, it's a feedback configuration. Under unity gain condition -- a well designed output stage assumed -- the ICMR (Input Common Mode Range) will determine the valid input range. So for a reasonable slew rate measurement, the input steps should be confined within this ICMR.
If you inject a full supply voltage range square wave input signal, the output slew rate most probably will be worse, as the opAmp needs time to reach a working OP (operation point) each time the input signal enters into the ICMR.