`test_block_with_wrong_slot` non-reproducible from fixture alone

The test run correctly fails in leanSpec because `BlockSpec.skip_slot_processing` results in `process_slots` to be skipped before block processing, but the emitted fixture doesn't include this information. Without it, test runners don't know for which tests they should skip slot processing.

This also happens with `test_block_at_parent_slot_rejected_when_slot_processing_skipped`.