Prevent index-only scans in stats regression test.

Prevent index-only scans in stats regression test.

This bollixes the test because it's expecting to see the idx_tup_fetch
counter increase, which won't happen if heap fetches were avoided by use
of an index-only scan.  Per buildfarm results.

While at it, let's just make sure that enable_seqscan and enable_indexscan
are ON for this test ...