Fix and re-enable test_finalize_validated.

[afck]

Motivation

This test is #[ignore]d because it's flaky: If the faulty validators respond before the other ones start processing the proposal, neither of them will have the proposal in their chain manager.

Also, test_sparse_sender_chain is flaky: If the randomly chosen validator (the first in the committee, i.e. by public key) is faulty, processing the notification will fail.

Proposal

Explicitly handle the proposal in one of the validators to make sure the test is set up for the intended scenario.

In test_sparse_sender_chain, use two validators with no faulty one, so the selected validator is guaranteed to have the block.

Test Plan

I ran it 20 times locally and it always passed.

Release Plan

• Nothing to do / These changes follow the usual release cycle.
• (Backporting would be no problem.)

Links

• Closes Fix test_finalize_validated #3860.
• reviewer checklist

[bart-linera]

🎉

[bart-linera]

Just as a note for the future: test_sparse_sender_chains needed fixing because it used one faulty validator, and after #4462 such validators have no chains. Before that PR, they used to have "fake" chains with zero initial balances, and because the test uses a chain that has a zero initial balance anyway, the faulty validator would still have the correct chain and the test would work even if it was chosen as the validator to use when processing the notification. Now, not having the chain, a faulty validator can't process the notification correctly, which is why the test became flaky and setting up the network without faulty validators fixes it.