An industry leader in test points out the key steps. Lincoln Lavoie at the University of New Hampshire, working with the Broadband Forum, runs the model test lab for DSL. They perform the crucial interoperability testing for the Forum. Working with the Forum, they brought an interop demo to BBWF, to show the world that G.fast chips are effectively communicating with each other.
Lavoie just published an article at Electronic Design, Best Practices for G.fast System Testing. He notes that both VDSL2 & G.fast incorporate "seamless rate adaption (SRA), bit swapping, retransmission, dying gasp, and vectoring," all of which need to be tested. Testing is also needed for G.fast features, "robust management channel (RMC), dynamic resource allocation (DRA), and fast rate adaption (FRA)."
Using a switch, noise generator, digital signal analyzer (signal-capture system), and Ethernet traffic generator, UNH can test most of the features of the equipment.
At BBWF, they used Telebyte and Digital Lightwave for the test gear as well as equipment based on chips from Metanoia, Sckipio, and Broadcom. A half dozen "plugfests" at UNH brought the vendors closer together, although all agree there's plenty of work to do.
In the article, Lincoln describes approaches to testing bit-swapping, seamless rate adoption, retransmission, and multi-line testing. There's also a helpful white paper from UNH.