tests: mem_protect/obj_validation: fix check for heap size

Commit 3fbf1248 introduced a way
to add to CONFIG_HEAP_MEM_POOL_SIZE if, for example, subsystems
need more heap space. However, the size check in obj_validation
was still using the old kconfig, and was failing when extra
space was specified. So update the size check to use the updated
size.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>