It's varied. Some posts below describe the standard software test engineering. Test engineers on Google Fiber would buy every microwave and 2.4GHz cordless phone and baby monitor, and see if our changes to interference mitigation algorithms improved or regressed between releases. So you're basically in a lab trying to break Wifi algorithms, probably not writing much code. (Also things like "does our change to move iPhone 6 to 5GHz when it's closer to the 5GHz access point also work with and iPhone 5?")