tests: aes.ofb: Prepare to char* to data_t* type change
tests: aes.ofb: Prepare to char* to data_t* type change

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 <b35ff5b7ca26c4ae9b66b92f2cf34224c8e26b08@arm.com>

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls