Fix engine tree tests with state root task enabled

[fgimenez]

Ref #14371

engine tree tests are failing when enabling state root task for instance https://github.com/paradigmxyz/reth/actions/runs/13244337100/job/36967035426

and locally:

✗ cargo t -p reth-engine-tree live_sync_fcu_extends_canon_chain -- --nocapture

running 1 test

thread 'tree::tests::test_engine_tree_live_sync_fcu_extends_canon_chain' panicked at crates/engine/tree/src/tree/mod.rs:3497:50:
called `Result::unwrap()` on an `Err` value: InsertBlockError { error: Provider(ConsistentView(Syncing { best_block: GotExpected { got: 0, expected: 0 } })), hash: 0xd9fcc226ccaf94a53ba8403e49d34999c09eda53045a1283a8e1d3a7f667d31e, number: 1, parent_hash: 0xcef5e0064593f96b34fd8b992f061f0862a15fe26cd3e29ebb4ece031b86c448, num_txs: 3, .. }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test tree::tests::test_engine_tree_live_sync_fcu_extends_canon_chain ... FAILED

failures:

failures:
    tree::tests::test_engine_tree_live_sync_fcu_extends_canon_chain

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 59 filtered out; finished in 0.47s

[fgimenez]

The unit tests use a MockEthProvider as the engine provider

reth/crates/engine/tree/src/tree/mod.rs

Line 3327 in 95b14f8

let provider = MockEthProvider::default();

and rely on its StateRootProvider impl to mock the state root calculation by setting the expected state root

reth/crates/engine/tree/src/tree/mod.rs

Line 3435 in 95b14f8

self.tree.provider.add_state_root(block.state_root);

with state root task this trait impl is not used, and the failure happens when trying to create a readonly database provider required for the consistent view that state root task uses:

reth/crates/storage/provider/src/test_utils/mock.rs

Lines 208 to 210 in 95b14f8

	fn database_provider_ro(&self) -> ProviderResult<Self::Provider> {
	Err(ConsistentViewError::Syncing { best_block: GotExpected::new(0, 0) }.into())
	}

to keep the unit tests isolated we need to be able to mock the state root task results. we could also have them less isolated and use a test db provider