test: fix flaky `test_subscribe_fails_causes_backup_to_be_used`

[0xNeshi]

test

Error: channel closed

https://github.com/OpenZeppelin/Event-Scanner/actions/runs/19225015176/job/54950193151

Problem seems to be that both Anvil nodes are started with Anvil::new().try_spawn()?, so maybe there's a conflict as the same instance is reused.

[0xNeshi]

Possibly related #167