Accept aggregation plan changes in xc_remote tests

The plans changed mainly due to abandoning the custom implementation
two-phase aggregation code, and using the upstream parallel aggregation.

That means we have stopped showing schema name in target lists, so
instead of

    Output: pg_catalog.avg((avg(xcrem_employee.salary)))

the EXPLAIN now shows

    Output: avg(xcrem_employee.salary)

and we also do projection at the scan nodes, so the target list only
shows the necessary subset of columns.

A somewhat surprising change is that the plans switch from distributed
aggregate plans like this one

    ->  Aggregate
        ->  Remote Subquery Scan
            ->  Aggregate
                -> Seq Scan

to always performing simple (non-distributed) aggregate like this

    ->  Aggregate
        ->  Remote Subquery Scan
            -> Seq Scan

This happens due to create_grouping_paths() relying on consider_parallel
flag when setting try_distributed_aggregate, disabling distributed
aggregation when consider_parallel=false. Both affected plans are however
for UPDATE queries, and PostgreSQL disables parallelism for queries that
do writes, so we end up with try_distributed_aggregate=false.

We should probably enable distributed aggregates in these cases, but we
can't ignore consider_parallel entirely, as we likely need some of the
checks. We will probably end up with consider_distributed flag, set in
a similar way to consider_parallel, but that's more an enhancement than
a bug fix.

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