Aircraft Engine Run-Ups are a part of one of their Checklists (Check - D i think), however, they are mostly done in a controlled facility called the "run-up bay" (The same is present at Karachi airport which can even be seen from the road.
Probabbly Sialkot Airport does not have a specialized facility for Run-Up testing of Engines, and they opted to perform the test on the apron itself (Probabbly using the Taxi-Way or Runway (closed for some time) would've been a wiser choice).
The following picture of the Run-up Bay at Khi airport, where you can see the floor design as well as the deflectors for deflecting the jet-wash. (Pic taken from net)
