It was not enabled, because there were a lot of errors in it, because of
features that have been disabled in Greenplum. Like enforcing foreign keys,
INSERT RETURNING, and doing SPI in functions in segments. Those limitations
make these tests a less interesting than in the upstream, but there are
still many good tests in there too. Like the test for anonymous DO blocks,
that are not tested by any other tests.

The .sql file was almost identical to the upstream version. Clean it up
to be even closer, by re-enabling some commented queries in it, removing
the duplicated tests for RETURN QUERY, removing the READS/CONTAINS/MODIFIES
SQL DATA noise words etc.

Memorize the expected output as it is. That's quite different from the
upstream, because of all the disabled functionality, but also because of
some cosmetic differences in error messages.

The 'schema_topology' test contained a copy of the for_vect(), so remove
that, now that we run the real thing.