The tests here use the test certificates from the cert module where
certificates are needed. Thus, wherever there is a dependency on
MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED, there is also a dependency on
MBEDTLS_CERTS_C.
Fixes `make test` when MBEDTLS_CERTS_C is disabled.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>