We want to test both sets of functions (ext and non-ext) in turn, so goto exit
is not really and option.
Also, separate setting up the context (which is going to be the same for both
ext and non-ext functions) from setting up the buffers (which will vary).
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>