Resolve most failures in stats_ext regression tests

This addresses simple failures in stats_ext regression tests, namely:

1) Failure to drop column that happens to be a distribution key of the
   table (picked automatically by CREATE TABLE). Resolved by explicitly
   distributing the table by another columns.

2) Commenting out DDL on FDWs, which are unsupported in Postgres-XL, and
   we already have this covered by plenty of other tests.

3) Commenting out a DO block, checking that we can't create statistics
   on a TOAST table. The error message will vary depending on what OID
   is assigned to the TOAST table. There's no nice way to catch that
   error in Postgres-XL, due to missing support for subtransactions.

4) Accept trivial plan changes, that simply add Remote Subquery into the
   plan, or ship the whole query to the nodes (Fast Query Execution).
   There additional plan changes that change the plans in other ways,
   but those need more investigation and are not accepted by this commit.

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