Fix initialization errors in unit tests

[goatgoose]

Resolved issues:

Resolves #4204.

Description of changes:

In some environments, some unit tests currently error with s2n not initialized when linked to the shared library of s2n-tls. For example, see the the result of this GH action that builds s2n-tls with a shared library on macOS before this fix was applied.

This error is due to including utils/s2n_mem.c in .c files:

s2n-tls/tests/testlib/s2n_mem_testlib.c

Lines 20 to 21 in 7f84701

	/* Required to override memory callbacks at runtime */
	#include "utils/s2n_mem.c"

When s2n_init() is called, initialized in s2m_mem.c is set to true in s2n_mem_init():

s2n-tls/utils/s2n_mem.c

Lines 249 to 253 in 7f84701

	int s2n_mem_init(void)
	{
	POSIX_ENSURE(s2n_mem_init_cb() >= S2N_SUCCESS, S2N_ERR_CANCELLED);
	initialized = true;

When s2n_mem.c is included from a .c file, this seems to cause initialized to be set to false when checked by functions such as s2n_free_without_wipe(), which is causing unit tests to incorrectly fail:

s2n-tls/utils/s2n_mem.c

Lines 287 to 291 in 7f84701

	int s2n_free_without_wipe(struct s2n_blob *b)
	{
	POSIX_PRECONDITION(s2n_blob_validate(b));
	POSIX_ENSURE(initialized, S2N_ERR_NOT_INITIALIZED);

This PR removes the s2n_mem.c include from .c files, and instead uses new get/override functions to get and set the mem callbacks.

Testing:

I modified the macOS GH action to additionally build and test the s2n-tls shared library, which fails without this fix.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.