Put back regression test case in a more robust form.

This undoes my hurried commit 776a2c88, restoring the removed test case
in a form that passes with or without force_parallel_mode = regress.

It turns out that force_parallel_mode = regress simply fails to mask
the Worker lines that will be produced by EXPLAIN (ANALYZE, VERBOSE).
I'd say that's a bug in that feature, as its entire alleged reason
for existence is to make the EXPLAIN output the same.  It's certainly
not a bug in the plan node pruning logic.  Fortunately, this test case
doesn't really need to use ANALYZE, so just drop that.

Discussion: https://postgr.es/m/18891.1576109690@sss.pgh.pa.us

src/test/regress/expected/partition_prune.out

@@ -3161,6 +3161,19 @@ execute mt_q1(35);
 (0 rows)
 
 deallocate mt_q1;
+prepare mt_q2 (int) as select * from ma_test where a >= $1 order by b limit 1;
+-- Ensure output list looks sane when the MergeAppend has no subplans.
+explain (verbose, costs off) execute mt_q2 (35);
+           QUERY PLAN           
+--------------------------------
+ Limit
+   Output: ma_test.a, ma_test.b
+   ->  Merge Append
+         Sort Key: ma_test.b
+         Subplans Removed: 3
+(5 rows)
+
+deallocate mt_q2;
 -- ensure initplan params properly prune partitions
 explain (analyze, costs off, summary off, timing off) select * from ma_test where a >= (select min(b) from ma_test_p2) order by b;
                                           QUERY PLAN                                           

src/test/regress/sql/partition_prune.sql

@@ -838,6 +838,13 @@ execute mt_q1(35);
 
 deallocate mt_q1;
 
+prepare mt_q2 (int) as select * from ma_test where a >= $1 order by b limit 1;
+
+-- Ensure output list looks sane when the MergeAppend has no subplans.
+explain (verbose, costs off) execute mt_q2 (35);
+
+deallocate mt_q2;
+
 -- ensure initplan params properly prune partitions
 explain (analyze, costs off, summary off, timing off) select * from ma_test where a >= (select min(b) from ma_test_p2) order by b;