As a rule (normal operation) it's best to avoid "lugging" (shift to a lower gear instead). But a brief test isn't going to hurt anything.
Breaking the crankshaft is normally associated with sustained high rpm operation, where the stresses on the crankshaft are much higher. Not just more power being produced (1500 rpm will only get you about 1/3 max power on a stock engine), but the greatly increased stress from having to stop and start each piston & rod assembly at the end of every stroke.
The TR2-4 crankshaft also suffers (according to Kas Kastner and others) from resonance effects. If you hang a bare crankshaft up and tap it lightly with a metal object, it will actually ring like a bell (this is an old-timer's trick for checking for cracks, a cracked crank won't ring). At various certain speeds, the impact from each cylinder firing can excite the resonance and, in effect, make it ring louder (more deflection). Added to the other stresses, it can flex the steel too far and eventually break it.
IIRC, Kas said that the first such danger point happens around 5200 rpm.