Add additional HMAC tests.

Not terribly important given that we already have NIST vectors, but may as
well. These tests come from upstream's
2cfbdd71dde0c3ddf4597eb20cc3e3fb8485fc15.

Change-Id: I4f8dadc7d5d1599d0b75ecdef06f2fc6a5cd8003
Reviewed-on: https://boringssl-review.googlesource.com/4962
Reviewed-by: Adam Langley <agl@google.com>