A few have questioned what difference running a jet at high power will make when leak checking.I can't comment on the 737 but this is the data I have from my training notes on the Airbus.
Airbus A320 powered by IAE V2500-A5 the oil pressure at idle is approx 160 psi. At 90% N2 the oil pressure becomes approx 300 psi, almost double.
Certain systems require High powered engine runs and these are completed in a run pen or a designated taxi way block at LHR.