filler: allow genesis state root calc to modify pre-alloc

[marioevz]

After Cancun fork, due to EIP-4788, t8n automatically updates the storage when calculating the genesis state root to perform the nonce increment of the beacon root stateful pre-compile:

"0x000000000000000000000000000000000000000b": {
                "nonce": "0x01",
                "balance": "0x00",
                "code": "0x",
                "storage": {}
            }

Instead of doing this manually for every test, with this PR we fetch the updated pre that is the result of calling the t8n to calculate the genesis state root, and use that as input pre for each test.

This has the added benefit that it will not matter if the client reading the genesis applies or not the pre-compile nonce update because it will be contained in the pre of the test.

[spencer-tb]

Very clean! :D