According to the Libra whitepaper:
In addition, a client can optionally create a replica of the entire database by synchronizing the
transaction history from the validators. While creating a replica, a client can verify that validators
executed transactions correctly, which increases accountability and transparency in the system. Other
clients can read from a client that holds a replica in the same way they would read from a validator
to verify the authenticity of the response. For the sake of simplicity, the rest of this paper assumes
that clients query a validator directly rather than a replica.
How can I begin to synchronize transaction history from testnet validators?