Simulate Headphone Load
A simple dummy-load to simulate a pair of headphones would be a plain-old resistor. 

Most headphones are 32Ω-600Ω, though occasionally a pair of "Phat Bass 30000 ultra super mega" or similar headphones will use 16Ω or even 8Ω drivers, to try to get more power from whatever device is driving them, to put out more volume.

If your system is passive, or cannot drive a 16Ω load without the output level dropping, You will have to place at least a unity-gain buffer in series with your headphone output. There is really no avoiding it.

I would recommend just taking 2 16Ω resistors, and soldering them across each channel of a TRS plug. That should work fine for verifying your circuit will work with ~99% of the headphones on the market.