When the catapult is ready, it will be used with deadweights for calibration hundreds of times or let me say several month before the first aircraft is shot.
The catapults built at Huangdicun should not be prototypes; there should be models of these same catapults that have already been tested at length at other sites in the country before having additional models built at Huangdicun which is a
naval air training facility for this competition.
I imagine after completion of construction of the two catapults they would've undergone initial test launches with deadweights to verify their uses, but relatively brief just to confirm everything is working as their previous models had worked, and then proceeded to potentially launch a fighter.
But all the above is irrelevant anyway -- you are thinking about this backwards.
The point is not to try and
prove whether a rumour
has occurred, that is the worst way of going about Chinese military watching when we depend on credible rumours as our life source, because we rarely get definitive confirmation.
Instead, the way about it is to see if there is ever any evidence that can
disprove a rumour
could've occurred, and in this case, one way of disproving cao gen's statement that a J-15 has undergone a launch at the EM catapult is by having satellite photos of Huangdicun NATF which do not show the catapults (or at least the EM catapult) looking like a state of completion within the last few days. OTOH if we happen to get photos that show the catapults are complete, then that would make the rumour plausible, which is an important step forwards in feeding us knowledge.