The query_correctness tests include some queries that have an invalid $lookup subpipeline. This is because those queries (when executed) may not ever actually reach the subpipeline to execute it; however the new 
OPTIMIZE_SUBPIPELINE_FOR_JOIN_OPT rule will optimize a subpipeline in any case, which results in those queries "failing" with a syntax error.
 
Given that these are not meaningful queries/ are incorrect, we should remove them.