Carry the execution payload through main's new fork-choice-vote test helper

Merging main brought in #552's `signed_block` test helper, written against a
`BlockBody` that has no `execution_payload`. This branch adds that field, so the
branch compiled and main compiled but their merge did not — a semantic conflict
with no textual conflict, which is why git reported the PR mergeable and only the
merge-commit build caught it.

The helper builds blocks for fork-choice tests that never reach the execution
layer, so the default all-zero payload is the right value.

Author: pablodeymo

crates/storage/src/store.rs

@@ -1909,6 +1909,7 @@ mod tests {
                 state_root: H256::ZERO,
                 body: BlockBody {
                     attestations: attestations.try_into().unwrap(),
+                    execution_payload: Default::default(),
                 },
             },
             proof: MultiMessageAggregate::default(),