You could use a general purpose step down transformer to test another transformer at less than full mains voltage. For example, a common 50VA 24VAC hvac control transformer could be used to power the transformer under test, output should the ratio of 24 / 120 * the expected output. Another, more desirable method is a variable transformer, a.k.a. Variac or Powerstat. Having one in an enclosure with outlet, switch and volt & ammeter is a great addition to any test bench.
All that said, my first thought is why bother. If there is no sign of over-temp or physical damage, an ohmmeter check should show open or shorted windings. A cheap meter will make checking for shorts more difficult but it can be done.
Use an inline switch with proper fusing and there is little to worry about. I have a switched, fused power strip on the bench with a big pilot light so I know when it’s on. Turn off, confirm, plug in, turn on. My usual configuration for testing a transformer with leads is a proper line cord set and wire nuts. PC mounts I always solder pins to a cord set. No alligator clips with mains. Period. Not worth the less than a minute it takes for either soldering or twisting wire nuts. I don’t fear HV, I respect it and know what it can do.
IMO, wiring direct to a plug and relying on an upstream circuit breaker will eventually lead to an unpleasant surprise when inserting that plug into an outlet.