Use MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_EPHEMERAL_ENABLED
instead of MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED to guard
code specific to the TLS 1.3 ephemeral key exchange mode.
Use it also for the dependencies of TLS 1.3 only tests
relying on ephemeral key exchange mode, but for
tests in tls13-kex-modes.sh where the change is done
later using all
MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_.*ENABLED macros.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>