Now that the 1.2.0-rc.2 spec contains the same `LightClientUpdate`
definition that Nimbus was already using before, the corresponding
SSZ test vectors can be re-enabled.
Incorporates the latest changes to the light client sync protocol based
on Devconnect AMS feedback. Note that this breaks compatibility with the
previous prototype, due to changes to data structures and endpoints.
See https://github.com/ethereum/consensus-specs/pull/2802