regression_4000: check if the generated DH private key is a CAAM black key

Check if the generated private DH key is a CAAM black key. If it is the
case, skip the buffer size check as the key size and CAAM black key
buffer size do not match.

Signed-off-by: Clement Faure <clement.faure@nxp.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>