It might be argued that P&HQ testing should, quite reasonably, to be the main province and specialism of the TP. Avionics and systems testing needs a TP, but perhaps can and should be much more under the planning and control of a specialist FTE? (By which I mean probably an avionics or systems engineer who has been ALSO trained in Flight Test practices and principles.)
But, let's face it, you need to get P&HQ right first, since if the flight platform aint up to the job, there's little point in pursuing the test programme (and as John quite rightly points out, is far more likely to lead to the loss of an aircraft than an avionics failure, particularly during flight test).
G
N.B. I'm in the middle of one of these at the moment, a test programme where we've hit some fundamental handling problems with the airframe, and have deliberately not progressed to any significant systems - or in our case primarily performance testing until we can make the aeroplane fly properly. Who would have thought that in the 21st century we'd hit aeroelastic aileron reversal of all things !