postgres_fdw: Fix test that didn't test what it claimed.

Antonin Houska reported that the planner does consider pushing
postgres_fdw_abs() to the remote side, which happens because we make
it shippable earlier in the test case file.

Jeevan Chalke provided this patch, which changes the join
condition to use random(), which is not shippable, instead.
Antonin reviewed the patch.

Discussion: http://postgr.es/m/15265.1511985971@localhost

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -3160,21 +3160,23 @@ drop operator public.<^(int, int);
 -- Input relation to aggregate push down hook is not safe to pushdown and thus
 -- the aggregate cannot be pushed down to foreign server.
 explain (verbose, costs off)
-select count(t1.c3) from ft1 t1, ft1 t2 where t1.c1 = postgres_fdw_abs(t1.c2);
-                                                QUERY PLAN                                                
-----------------------------------------------------------------------------------------------------------
+select count(t1.c3) from ft2 t1 left join ft2 t2 on (t1.c1 = random() * t2.c2);
+                                        QUERY PLAN                                         
+-------------------------------------------------------------------------------------------
  Aggregate
    Output: count(t1.c3)
-   ->  Nested Loop
+   ->  Nested Loop Left Join
          Output: t1.c3
-         ->  Foreign Scan on public.ft1 t2
-               Remote SQL: SELECT NULL FROM "S 1"."T 1"
+         Join Filter: ((t1.c1)::double precision = (random() * (t2.c2)::double precision))
+         ->  Foreign Scan on public.ft2 t1
+               Output: t1.c3, t1.c1
+               Remote SQL: SELECT "C 1", c3 FROM "S 1"."T 1"
          ->  Materialize
-               Output: t1.c3
-               ->  Foreign Scan on public.ft1 t1
-                     Output: t1.c3
-                     Remote SQL: SELECT c3 FROM "S 1"."T 1" WHERE (("C 1" = public.postgres_fdw_abs(c2)))
-(11 rows)
+               Output: t2.c2
+               ->  Foreign Scan on public.ft2 t2
+                     Output: t2.c2
+                     Remote SQL: SELECT c2 FROM "S 1"."T 1"
+(13 rows)
 
 -- Subquery in FROM clause having aggregate
 explain (verbose, costs off)

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -829,7 +829,7 @@ drop operator public.<^(int, int);
 -- Input relation to aggregate push down hook is not safe to pushdown and thus
 -- the aggregate cannot be pushed down to foreign server.
 explain (verbose, costs off)
-select count(t1.c3) from ft1 t1, ft1 t2 where t1.c1 = postgres_fdw_abs(t1.c2);
+select count(t1.c3) from ft2 t1 left join ft2 t2 on (t1.c1 = random() * t2.c2);
 
 -- Subquery in FROM clause having aggregate
 explain (verbose, costs off)