tests: nist_kw: Prepare to char* to data_t* type change
tests: nist_kw: Prepare to char* to data_t* type change

In preparation of changing the type of some parameters
of mbedtls_nist_kw_wrap/unwrap() 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.

Signed-off-by: Ronald Cron <b35ff5b7ca26c4ae9b66b92f2cf34224c8e26b08@arm.com>

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