Write unit tests for all constant-time functions

[gilles-peskine-arm]

All non-static functions in constant_time.c should have unit tests checking that they are functionally correct. Use TEST_CF_SECRET and TEST_CF_PUBLIC adequately so that the all.sh components test_*_constant_flow check that the runs are constant-time.

The definition of done for this issue is that all functions declared in include/mbedtls/constant_time.h, library/constant_time_internal.h or library/constant_time_invasive.h have unit tests, except for mbedtls_ct_rsaes_pkcs1_v15_unpadding which has its own issue.

This can be done one function at a time. All the functions concerned by this issue have fairly simple semantics, so the difficulty will be mainly in adding appropriate TEST_CF annotations and choosing test data.

As of 52f83dc471625f503a335ccef4f4b484a2a579ef, constant_time.c exports following functions:

Library function	Test function
mbedtls_ct_uint_mask
mbedtls_ct_size_mask
mbedtls_ct_mpi_uint_mask
mbedtls_ct_size_mask_ge
mbedtls_ct_size_bool_eq
mbedtls_ct_mpi_uint_lt
mbedtls_mpi_core_lt_ct	mpi_core_lt_ct (test_suite_mpi)
mbedtls_ct_uint_if
mbedtls_ct_mpi_uint_cond_assign
mbedtls_ct_base64_enc_char	enc_chars (test_suite_base64)
mbedtls_ct_base64_dec_value	dec_chars (test_suite_base64)
mbedtls_ct_memcpy_if_eq
mbedtls_ct_memcpy_offset	ssl_cf_memcpy_offset (test_suite_ssl)
mbedtls_ct_hmac	ssl_cf_hmac (test_suite_ssl)
mbedtls_ct_rsaes_pkcs1_v15_unpadding	https://github.com/Mbed-TLS/mbedtls/issues/6272