deprecate stderr usage in our units tests

[jmayclin]

Problem:

Ctest does not play well with stderr. Even when enabling --output-on-failure I still don't see the output of our fprintf statements, like the one below

s2n-tls/tests/unit/s2n_resume_test.c

Lines 101 to 104 in 20b0174

	if (expected_size != actual_size) {
	fprintf(stderr, "\nS2N_TLS12_STATE_SIZE_IN_BYTES (%i) should be set to %u\n\n",
	S2N_TLS12_STATE_SIZE_IN_BYTES, expected_size);
	}

If I change stderr to stdout I do see the output.

Solution:

use printf instead.

Requirements / Acceptance Criteria:

error message should show up in ctest when --output-on-failure is enabled.