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 ...

src/test/regress/expected/stats.out

@@ -11,6 +11,11 @@ SHOW track_counts;  -- must be on
  on
 (1 row)
 
+-- ensure that both seqscan and indexscan plans are allowed
+SET enable_seqscan TO on;
+SET enable_indexscan TO on;
+-- for the moment, we don't want index-only scans here
+SET enable_indexonlyscan TO off;
 -- wait to let any prior tests finish dumping out stats;
 -- else our messages might get lost due to contention
 SELECT pg_sleep(2.0);

src/test/regress/sql/stats.sql

@@ -8,6 +8,12 @@
 -- conditio sine qua non
 SHOW track_counts;  -- must be on
 
+-- ensure that both seqscan and indexscan plans are allowed
+SET enable_seqscan TO on;
+SET enable_indexscan TO on;
+-- for the moment, we don't want index-only scans here
+SET enable_indexonlyscan TO off;
+
 -- wait to let any prior tests finish dumping out stats;
 -- else our messages might get lost due to contention
 SELECT pg_sleep(2.0);