In preparation of changing the type of some parameters
of aes_encrypt_ofb() from `char *` to `data_t` to get rid
of the calls to mbedtls_test_unhexify():
- Change the name of parameters and local variables to
clarify which ones are related to the outputs of the
library functions under test and which ones are
related to the expected values of those outputs.
- Add assertion on fragment_size parameter
Signed-off-by: Ronald Cron <ronald.cron@arm.com>