feat(txpool): handle more simulated scenarios in test_utils/pool.rs

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• implements ScenarioType::HigherNonce with unit test
• implements ScenarioType::BelowBaseFee with unit test
• previously MockPool::default() created a pool with pending_basefee = MIN_PROTOCOL_BASE_FEE and it wasn't matching the simulation's base_fee so also added create_pool() to align simulation's base_fee with pool's pending_basefee.

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@mattsse this is still missing dynamic promotion tests, following previous state, but i think the foundation should be working

the goal now is to use tracked state to trigger promotions programatically

wdyt

[mattsse]

cool, yep, this lgtm