Considering that the HMAC(SHA-512) DRBG is the default DRBG now, a self
test is to be provided.

The test vector is obtained from a successful NIST ACVP test run.
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>