test on J-20. The last time single engine test was performed in the same way was WS-10 on J-11. Only brand new engine is tested this way due to concern of reliability of a newly designed engine.

See bomberman's post: J-10B Cobra. TVC WS-10 installed on single engine J-10 show confidence in its reliability. TVC demonstrated in Zhuhai was not a test flight, it's a show off of the mastery of thrust vector engine and flight control.