Force TLS 1.2 version on tests related to
MBEDTLS_SSL_ASYNC_PRIVATE, CA callback and
MBEDTLS_SSL_MAX_FRAGMENT_LENGTH. Those
SSL options are not supported in TLS 1.3
for the time being. Thus force TLS 1.2
version in preparation of TLS 1.3 being
the default protocol version when both
TLS 1.2 and TLS 1.3 are enabled.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>