Improve stability of recently-added regression test case.

Commit b5febc1d added a contrib/btree_gist test case that has been
observed to fail in the buildfarm as a result of background auto-analyze
updating stats and changing the selected plan.  Forestall that by
forcibly analyzing in foreground, instead.  The new plan choice is
just as good for our purposes, since we really only care that an
index-only plan does not get selected.

Back-patch to 9.5, like the previous patch.

Discussion: https://postgr.es/m/14643.1539629304@sss.pgh.pa.us

contrib/btree_gist/expected/inet.out

@@ -64,18 +64,16 @@ SELECT count(*) FROM inettmp WHERE a >  '89.225.196.191'::inet;
    386
 (1 row)
 
-VACUUM inettmp;
+VACUUM ANALYZE inettmp;
 -- gist_inet_ops lacks a fetch function, so this should not be index-only scan
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM inettmp WHERE a  = '89.225.196.191'::inet;
                     QUERY PLAN                    
---------------------------------------------------------
+--------------------------------------------------
  Aggregate
-   ->  Bitmap Heap Scan on inettmp
-         Recheck Cond: (a = '89.225.196.191'::inet)
-         ->  Bitmap Index Scan on inetidx
+   ->  Index Scan using inetidx on inettmp
          Index Cond: (a = '89.225.196.191'::inet)
-(5 rows)
+(3 rows)
 
 SELECT count(*) FROM inettmp WHERE a  = '89.225.196.191'::inet;
  count 
@@ -89,13 +87,11 @@ CREATE INDEX ON inettmp USING gist (a gist_inet_ops, a inet_ops);
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM inettmp WHERE a  = '89.225.196.191'::inet;
                      QUERY PLAN                     
---------------------------------------------------------
+----------------------------------------------------
  Aggregate
-   ->  Bitmap Heap Scan on inettmp
-         Recheck Cond: (a = '89.225.196.191'::inet)
-         ->  Bitmap Index Scan on inettmp_a_a1_idx
+   ->  Index Scan using inettmp_a_a1_idx on inettmp
          Index Cond: (a = '89.225.196.191'::inet)
-(5 rows)
+(3 rows)
 
 SELECT count(*) FROM inettmp WHERE a  = '89.225.196.191'::inet;
  count 

contrib/btree_gist/sql/inet.sql

@@ -30,7 +30,7 @@ SELECT count(*) FROM inettmp WHERE a >= '89.225.196.191'::inet;
 
 SELECT count(*) FROM inettmp WHERE a >  '89.225.196.191'::inet;
 
-VACUUM inettmp;
+VACUUM ANALYZE inettmp;
 
 -- gist_inet_ops lacks a fetch function, so this should not be index-only scan
 EXPLAIN (COSTS OFF)