In test functions calling mbedtls_test_unhexify(), change the
type of the associated parameters from `char*` to `data_t`.
That way the `unhexify` operation is done by the test
framework and not by the unit test code.
Use for the new parameters of type data_t the name of the
local variable that use to store the `unhexify` version of
the `char*` parameter.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>