tests: mbedtls: Call mbedtls_platform_set_printf() only if needed

It exists only if MBEDTLS_PLATFORM_PRINTF_ALT is defined, whcih we
no longer do, but make the call condiitonal in case we'll need to
make it configurable later.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>