Fix flakey percentile test

This query was added to "percentile" in commit b877cd43 as outer references
were allowed after we backported ordered-set aggregates. But the query was
using an ARRAY sublink, which semantically does not guarantee ordering. This
causes sporadic failure in tests.

This commit tweaks the test query so that it has a deterministic ordering in
the output array.

We considered just adding an ORDER BY to the subquery, but ultimately we chose
to use `array_agg` with an `ORDER BY` because subquery order is not preserved
per SQL standard.
Signed-off-by: NSambitesh Dash <sdash@pivotal.io>

src/test/regress/expected/percentile.out

@@ -1131,10 +1131,10 @@ select (select a from perct where median(t.a) = 5) from perct t;
          
 (1 row)
 
-select array((select a from perct where median(t.a) = 50.5)) from (select * from perct t order by a offset 0) as t;
+select (select array_agg(a ORDER BY a) from perct where median(t.a) = 50.5) from (select * from perct t order by a offset 0) as t;
                                                                                                                                                ?column?                                                                                                                                                
 -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
- {1,2,13,14,15,16,17,28,29,30,31,33,34,35,36,37,48,49,50,51,52,63,68,69,70,71,72,83,84,85,86,3,4,5,6,7,18,19,20,21,22,38,39,40,41,42,53,54,55,56,57,73,74,75,76,77,87,88,89,90,91,96,97,8,9,10,11,12,23,24,25,26,27,32,43,44,45,46,47,58,59,60,61,62,64,65,66,67,78,79,80,81,82,92,93,94,95,98,99,100}
+ {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100}
 (1 row)
 
 -- MPP-22219

src/test/regress/sql/percentile.sql

@@ -223,7 +223,7 @@ select percentile_cont(now()) within group (order by a) from percts;
 select percentile_cont(0.5) within group (order by point(0,0)) from perct;
 -- outer references
 select (select a from perct where median(t.a) = 5) from perct t;
-select array((select a from perct where median(t.a) = 50.5)) from (select * from perct t order by a offset 0) as t;
+select (select array_agg(a ORDER BY a) from perct where median(t.a) = 50.5) from (select * from perct t order by a offset 0) as t;
 
 -- MPP-22219
 select count(*) from