Flight testing should primarily be for airframe integration testing and subsequent optimization. That comes near the end or completion of production prototype testing. When China was flight testing the WS-10 on a flying testbed and then a J-11 they didn’t have the facilities to simulate different air conditions and didn’t have confidence or experience with modern engine testing methods, but normally these tests are done on a platform in specialized wind tunnels. Turnaround times with the latter method is much shorter, and it involves a lot less work with regards to maintenance, design revision, and setup. China’s engine testing infrastructure has become a lot more developed, so it’s unlikely we’ll see the WS-15 go onto a flying test bed the way we say the WS-10. By the time it’s flying in the air it will probably be in one of the prototype J-20s doing the aforementioned integration testing.
And yes, that image is a test prototype of the WS-19.
