Fix recently-added array_agg tests to ensure they produce stable results

regardless of plan changes.  Per intermittent buildfarm failures on
"pigeon" and others.

src/test/regress/expected/arrays.out

@@ -1125,22 +1125,22 @@ select c, cardinality(c), d, cardinality(d) from arrtest;
  {foo,new_word}    |           2 | {bar,foo}     |           2
 (3 rows)
 
-select array_agg(unique1) from tenk1 where unique1 < 15;
+select array_agg(unique1) from (select unique1 from tenk1 where unique1 < 15 order by unique1) ss;
               array_agg               
 --------------------------------------
- {4,2,1,6,14,9,8,5,3,13,12,11,7,10,0}
+ {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14}
 (1 row)
 
-select array_agg(ten) from tenk1 where unique1 < 15;
+select array_agg(ten) from (select ten from tenk1 where unique1 < 15 order by unique1) ss;
             array_agg            
 ---------------------------------
- {4,2,1,6,4,9,8,5,3,3,2,1,7,0,0}
+ {0,1,2,3,4,5,6,7,8,9,0,1,2,3,4}
 (1 row)
 
-select array_agg(nullif(ten, 4)) from tenk1 where unique1 < 15;
+select array_agg(nullif(ten, 4)) from (select ten from tenk1 where unique1 < 15 order by unique1) ss;
                array_agg               
 ---------------------------------------
- {NULL,2,1,6,NULL,9,8,5,3,3,2,1,7,0,0}
+ {0,1,2,3,NULL,5,6,7,8,9,0,1,2,3,NULL}
 (1 row)
 
 select cardinality(array_agg(unique1)) from tenk1 where unique1 < 15;
@@ -1149,12 +1149,6 @@ select cardinality(array_agg(unique1)) from tenk1 where unique1 < 15;
           15
 (1 row)
 
-select array_agg(unique1) from (select * from tenk1 order by unique1 asc) as tab where unique1 < 15;
-              array_agg               
---------------------------------------
- {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14}
-(1 row)
-
 select array_agg(unique1) from tenk1 where unique1 < -15;
  array_agg 
 -----------

src/test/regress/sql/arrays.sql

@@ -396,11 +396,10 @@ select cardinality(array[1,2,3]);
 select cardinality(array[[1,2,3], [4,5,6]]);
 select c, cardinality(c), d, cardinality(d) from arrtest;
 
-select array_agg(unique1) from tenk1 where unique1 < 15;
-select array_agg(ten) from tenk1 where unique1 < 15;
-select array_agg(nullif(ten, 4)) from tenk1 where unique1 < 15;
+select array_agg(unique1) from (select unique1 from tenk1 where unique1 < 15 order by unique1) ss;
+select array_agg(ten) from (select ten from tenk1 where unique1 < 15 order by unique1) ss;
+select array_agg(nullif(ten, 4)) from (select ten from tenk1 where unique1 < 15 order by unique1) ss;
 select cardinality(array_agg(unique1)) from tenk1 where unique1 < 15;
-select array_agg(unique1) from (select * from tenk1 order by unique1 asc) as tab where unique1 < 15;
 select array_agg(unique1) from tenk1 where unique1 < -15;
 
 select unnest(array[1,2,3]);