Get rid of MBEDTLS_TEST_DEFINES_ZEROIZE

[gilles-peskine-arm]

In include/mbedtls/platform_util.h, since #8143, we skip the declaration of mbedtls_platform_zeroize in some test builds, based on whether MBEDTLS_TEST_DEFINES_ZEROIZE is defined. There's a good reason for wanting to skip the declaration: we want to make mbedtls_platform_zeroize a macro so that we can benefit from a compiler warning (see discussion).

The current situation is unfortunate for two reasons. One reason is that it's generally error-prone to have differences in how the product behave based on whether it's being tested, and what we're doing here breaks our invasive testing guidelines. Another more specific reason is that it would be good to have this compiler warning in production builds on platforms where it's possible. I'm not sure what these platforms are, but hopefully platforms with memset_s could have both a good zeroize function and a warning from GCC and Clang.

So we should redesign the platform configuration for mbedtls_platform_zeroize to allow production builds to have the warning. We noted in the discussion at the time that we should do that when we redesign the platform interface, which we are doing now for 4.0.