Failures with installcheck and low work_mem value in 13~

Hi all,

Attempting to run installcheck with 13~ and a value of work_mem lower
than the default causes two failures, both related to incremental
sorts (here work_mem = 1MB):
1) Test incremental_sort:
@@ -4,12 +4,13 @@
 select * from (select * from tenk1 order by four) t order by four, ten;
             QUERY PLAN
 -----------------------------------
- Sort
+ Incremental Sort
    Sort Key: tenk1.four, tenk1.ten
+   Presorted Key: tenk1.four
    ->  Sort
          Sort Key: tenk1.four
          ->  Seq Scan on tenk1
-(5 rows)
+(6 rows)

2) Test join:
@@ -2368,12 +2368,13 @@
    ->  Merge Left Join
          Merge Cond: (x.thousand = y.unique2)
          Join Filter: ((x.twothousand = y.hundred) AND (x.fivethous = y.unique2))
-         ->  Sort
+         ->  Incremental Sort
                Sort Key: x.thousand, x.twothousand, x.fivethous
-               ->  Seq Scan on tenk1 x
+               Presorted Key: x.thousand
+               ->  Index Scan using tenk1_thous_tenthous on tenk1 x
          ->  Materialize
                ->  Index Scan using tenk1_unique2 on tenk1 y
-(9 rows)
+(10 rows)

There are of course regression failures when changing the relation
page size or such, but we should have tests more portable when it
comes to work_mem (this issue does not exist in ~12) or people running
installcheck on a new instance would be surprised.  Please note that I
have not looked at the problem in details, but a simple solution would
be to enforce work_mem in those code paths to keep the two plans
stable.

Thanks,
--
Michael