tests: chacha20: Prepare to char* to data_t* type change
tests: chacha20: Prepare to char* to data_t* type change

In preparation of changing the type of some parameters of
test_chacha20() from `char *` to `data_t` to get rid of the
calls to mbedtls_test_unhexify():

- Reduce the size of output[] buffer to 375 as its content
  is "ASCII expended" into a buffer of 751 bytes.
- Align naming of variables to store and check the
  output of mbedtls_chacha20_crypt(). No *dst* variables
  anynore, only *output* variables.
- Use two different buffers to store the expected output
  of mbedtls_chacha20_crypt() (expected_output_str[]) and
  the ASCII string representation of the output of
  mbedtls_chacha20_crypt() (output_string[]). Both were
  stored in dst_str[] before.

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

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