I would have imagined that all the main flight testing programs would have finalised many years ago. Any more flight testing would be related to actual operations and developing tactics which should be done all over the place by J-20 pilots. It doesn't make sense to fly J-20 to where WS-15 is but I don't think that's what is actually happening. This cartoon suggests some link between J-20 and Xian that's its main purpose. I don't think it indicates a literal meaning - J-20 flying to Xian. Rather suggesting the link between the two serves to hint at something from Xian, not necessarily something that will remain in Xian. Don't know if I've explained it well.
Also the markings seem to show a significant change to J-20. Typical flight testing usually does have it as well like on the J-15 but the time for that phase of J-20 development is far behind us. The pattern here has been markings on J-10 and new engine (TVC one) being tested even though the actual J-10/s that had been testing new TVC engines were not photographed with those markings. Therefore it's possible those marking on the cartoon are to suggest new engine since that's been the pattern so far, albeit a questionable one. I think it's more likely this is hinting at WS-15 + J-20 connection rather than J-20 going to flight testing in Xian.
I imagine most of the flight testing should have been done as well, with Al-31s at least. But it could be referencing the WS-10 equipped J-20s going to Yanliang.
But if the image is meant to be taken figuratively rather than literally then I suppose one could interpret it to possibly suggest some kind of relationship between J-20 and "Xi'an" which one could stretch to mean WS-15 if one really wanted to, but it's really a stretch.
Either way, I see nothing compelling to suggest that it is literally or figuratively suggesting that it's suggesting that J-20 will be flying with WS-15 soon. At best, we can't rule out that it may be what the picture is trying to convey, but there's far from anything convincing in the image to suggest it imo