Crypto: Continue sign message test if not supported

The sign/verify message test might continue in case the
algorithm is not supported for signing but could still
be verified, i.e. for those cases where deterministic
ECDSA is not supported but normal ECDSA is.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I05bdcdaf6626b8f65bf04c66ae33a979c6106aed