Adjust regression test cases added by commit ab727167.

I suppose it is a copy-and-paste error that this test doesn't actually
test the "Parallel Append with both partial and non-partial subplans"
case (EXPLAIN alone surely doesn't qualify as a test of executor
behavior).  Fix that.

Also, add cosmetic aliases to make it possible to tell apart these
otherwise-identical test cases in log_statement output.

src/test/regress/expected/select_parallel.out

@@ -29,7 +29,7 @@ explain (costs off)
                      ->  Parallel Seq Scan on f_star
 (11 rows)
 
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a1;
  round | sum 
 -------+-----
     14 | 355
@@ -55,6 +55,12 @@ explain (costs off)
                      ->  Parallel Seq Scan on f_star
 (11 rows)
 
+select round(avg(aa)), sum(aa) from a_star a2;
+ round | sum 
+-------+-----
+    14 | 355
+(1 row)
+
 -- Parallel Append with only non-partial subplans
 alter table a_star set (parallel_workers = 0);
 alter table b_star set (parallel_workers = 0);
@@ -77,7 +83,7 @@ explain (costs off)
                      ->  Seq Scan on a_star
 (11 rows)
 
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a3;
  round | sum 
 -------+-----
     14 | 355
@@ -108,7 +114,7 @@ explain (costs off)
                      ->  Parallel Seq Scan on f_star
 (11 rows)
 
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a4;
  round | sum 
 -------+-----
     14 | 355

src/test/regress/sql/select_parallel.sql

@@ -18,13 +18,14 @@ set max_parallel_workers_per_gather=4;
 -- Parallel Append with partial-subplans
 explain (costs off)
   select round(avg(aa)), sum(aa) from a_star;
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a1;
 
 -- Parallel Append with both partial and non-partial subplans
 alter table c_star set (parallel_workers = 0);
 alter table d_star set (parallel_workers = 0);
 explain (costs off)
   select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a2;
 
 -- Parallel Append with only non-partial subplans
 alter table a_star set (parallel_workers = 0);
@@ -33,7 +34,7 @@ alter table e_star set (parallel_workers = 0);
 alter table f_star set (parallel_workers = 0);
 explain (costs off)
   select round(avg(aa)), sum(aa) from a_star;
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a3;
 
 -- Disable Parallel Append
 alter table a_star reset (parallel_workers);
@@ -45,7 +46,7 @@ alter table f_star reset (parallel_workers);
 set enable_parallel_append to off;
 explain (costs off)
   select round(avg(aa)), sum(aa) from a_star;
-select round(avg(aa)), sum(aa) from a_star;
+select round(avg(aa)), sum(aa) from a_star a4;
 reset enable_parallel_append;
 
 -- test with leader participation disabled