Simulating wi-fi client traffic can be done with mac with the Developer tool Link Conditioner.. This is very handy if you want to test lossy traffic conditions. You can find this under Xcode > Open Developer Tool > More Developer Tools.
There are a couple of commercial network traffic simulation tools for Wi-Fi clients but they are VERY expensive. Normally they are used by WLAN vendors to test their equipment, rarely are they used to test WLAN deployments.