tests: mbedtls: Fix GCC warning about `test_snprintf`

Fix errors like:

inlined from ‘test_mbedtls’ at
zephyrproject/zephyr/tests/crypto/mbedtls/src/mbedtls.c:172:6:
zephyrproject/zephyr/tests/crypto/mbedtls/src/mbedtls.c:96:17: error:
‘test_snprintf’ reading 10 bytes from a region of size 1
[-Werror=stringop-overread]
   96 |                 test_snprintf(1, "", -1) != 0 ||
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~

In GCC >= 11 because `ret_buf` in some calls are shorter literals

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>