Your goal is to test the amplifier, but your question is how to calculate the value and power for a resistor in series with a under-dimensioned test speaker. If I were you, and really wanted to test the amplifier, I would so as suggested by others to only use a power resistor. In addition I would thrown in a 0.5 mH inductor as well (rated for the current used). I would probably go for 5 ohms resistor value, because that is most typical. In addition I would throw in a passive RLC network to emulate the phaseshifts you'll find in a speaker with passive crossover networks.
Test with real signals, white noise and single frequencies. Test also with and without the RLC network. Then you'll get fair idea of how the amplifier behaves under different kinds of loads. As always you should do the same with an amplifier you know, because you need some kind of reference in order to judge the results.