maybe you don't know how but how does youtube and netflix work then because it does not mirror the device
What newt said is accurate; Chromecast is not a full-fledged Android device out of the box and can only run a small subset of apps. Streaming video from a service is not the same thing as running a full-featured Android app.
If you're trying to test for future usage on Nexus TV, and assuming that's a full-fledged Android device, something like the MK808B or other, newer Android PC on-a-stick devices could serve as a potential testing platform - assuming, again, that the Nexus TV will be a full Android install with Google Play support. Those Android PCs are pretty powerful for their cost ($40ish-$100ish), and also serve as a good testbed for Ouya and Gamestick.