Add server-side FallbackSCSV tests.

Assert that inappropriate fallbacks are detected, but if the client_version
matches the server's highest version, do not abort the handshake.

Change-Id: I9d72570bce45e1eb23fc2b74a3c5fca10562e573
Reviewed-on: https://boringssl-review.googlesource.com/1373
Reviewed-by: Adam Langley <agl@google.com>