Maintain a handshake buffer in prf.go to implement TLS 1.2 client auth. Also
use it for SSL 3. This isn't strictly necessary as we know the hash functions,
but Go's hash.Hash interface lacks a Copy method.
Also fix the server-side tests which failed to test every TLS version.
Change-Id: I98492c334fbb9f2f0f89ee9c5c8345cafc025600
Reviewed-on: https://boringssl-review.googlesource.com/1664
Reviewed-by: Adam Langley <agl@google.com>
e098ec2460