[test] Add HMAC self-tests

[test] Add HMAC self-tests

The HMAC code is already tested indirectly via several consuming
algorithms that themselves provide self-tests (e.g. HMAC-DRBG, NTLM
authentication, and PeerDist content identification), but lacks any
direct test vectors.

Add explicit HMAC tests and ensure that corner cases such as empty
keys, block-length keys, and over-length keys are all covered.

Signed-off-by: Michael Brown <mcb30@ipxe.org>