Fix test_m005_v3 sensor descriptor parsing

SCMIv3.0 Sensor descriptors have variable lengths based on the presence
or not of extended attributes; the presence of extended attributes is
NOT mandatory though even in SCMIv3.0 messages, so that, within the same
reply you could received a mix of sensor descriptors carrying or NOT the
extended attributes: as a consequence the array of sensor descriptors is
composed of variable length elements and as such must be parsed.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>