test: cedit: use allocated address for reading file

Instead of a randomly selected address, use an LMB allocated one for
reading the file into memory. With the LMB map now being persistent
and global, the address used for reading the file might be already
allocated as non-overwritable, resulting in a failure. Get a valid
address from LMB and then read the file to that address.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>