test(slot buffer): add assertion tests

This patch also fixes the dest and src addresses not being checked for
ns_buffer_read() and ns_buffer_write() respectively as well as some
lines exceeding 80 cols on granule.cpp

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I71cac6e85294fac828c950b227cf9d9fb27164ae