As C52 said asymmetry may pose an interesting challenge? Particularly with a potentially longer thrust arm with the test engine being mounted fairly far forward.
I'd guess that C52 meant asymmetry in the sense of structural loads on the fuselage, rather than handling characteristics.
In the case of the latter, I can't see there being much problem as the GTF is no further outboard than No 2/3 engines, and producing much less thrust.