I always test my high level alarm with actual high water level. It is easy, I just unplug the bilge pump while doing a load of laundry :) It is a good time to test the manual bilge pump too.
If you rely on a lever to lift the float as your test, it is true you test most parts of the system, but what happens if the float has cracked and no longer floats? Your test would pass, but...
Big Grand Cay, Abacos, Bahamas