The engine that you want to install on aircraft is supposed to be good: I assume (I might be wrong?) this engine was overhauled or repaired and tested in test cell in engines shop especially if internal items of engine were replaced. Either the engine is overhauled/repaired or new, it will require only few tests (leak test, gen disconnect,...) and not hours of high power engine run. May be you are using engine aircraft run as a substitute to engine test cell?
I hope I can get some clarifications regarding this subject: I guess I am missing or mis-understanding something especially when it come to engines shop work and Hangar or line work.
The RB211 is a modular engine. There are 7 modules including the fan case. When the engine is taken into the shop a decision is made of which modules to overhaul. In the early days (I am talking 1977) the HP Turbine had a very limited life. So every 1100 or so cycles the engine was removed to the shop. If there was no other problem, the shop could replace the HP (04) module in a couple of days. We then sent the removed 04 module off for overhaul, and put the repaired engine onto the next aircraft that needed one. This engine then needed a test bed acceptance run. In the RR MM are a list of tests numbered 1 to 17. The manual tells you which test to do depending on what has been changed on the engine. Test 1 was oil circulation and fuel drains, test 17 was Thrust reverser function. The important tests were test 4 and 5 which were running in the engine, and checking performance. We then converted all the figures back to ISA and calculated the engine performance. On the RB211-22B it was sometimes marginal on installation!
If you did these tests in order it could take some time, but having a lot of practice I could easily do the lot under the hour. My personal record was a Nbr 3 engine. Aircraft came in the hangar at 1400, on the run bay at 1820, and on the ramp at 2000 for a 2130 departure. But we were waiting for it, and the new engine was a good one. The Nbr 2 engine took a lot longer!