Testutil server with premined addresses

[ferranbt]

The current testutil does not premine any accounts at genesis and uses the dev unlocked accounts to send transactions and fund other accounts with the eth_sendTransaction endpoint. There are two main reasons to move away from this workflow:

• It is not sure how long the eth_sendTransaction endpoint will be available (i.e. neither Erigon nor Polygon-Edge includes it).
• It requires extra blocks and time to fund the accounts.

The work would require to use a custom genesis file to setup the premined accounts and use it instead of the bultin genesis file for dev.