TestContext sets tx nonces incorrectly (sometimes)

[ed255]

The TestContext is used to setup an Ethereum state, run txs on it with go-ethereum and get the resulting traces. When using various transactions, the TestContext automatically assigns a default nonce to them, in sequence starting from 0 (and these values can then be overwritten by the user). This only works assuming all txs come from the same address and this address has nonce 0 at the beginning:

zkevm-circuits/mock/src/test_ctx.rs

Lines 135 to 146 in 06cce9f

	// By default, set the TxIndex and the Nonce values of the multiple transactions
	// of the context correlative so that any Ok test passes by default.
	// If the user decides to override these values, they'll then be set to whatever
	// inputs were provided by the user.
	transactions
	.iter_mut()
	.enumerate()
	.skip(1)
	.for_each(|(idx, tx)| {
	let idx = u64::try_from(idx).expect("Unexpected idx conversion error");
	tx.transaction_idx(idx).nonce(idx);
	});

This automatic assignment of nonces doesn't work for example when you have 2 txs from different accounts (each with nonce = 0). I think the automatic assignment should only be implemented it if covers all cases, because otherwise the developer may take some time to realize what's failing: When having 2 txs from different accounts the developer may assume that each tx.nonce is 0 (because that's the default value of the MockTransaction), but they will find that the second tx is sent with nonce = 1.

So I think we should either remove this automatic (sometimes incorrect) nonce assignment, or alternatively do it right (by inspecting the from address, and checking the nonces in the setup accounts).

[zemse]

Thanks for marking this as good first issue. I am interested in taking this up.