Add proper regression test for the recent SRFs-in-pathkeys problem.

Remove the test case added by commit fac1b470a, which never actually
worked to expose the problem it claimed to test.  Replace it with
a case that does expose the problem, and also covers the SRF-not-
at-the-top deficiency repaired in 1aa8dad41.

Richard Guo, with some editorialization by me

Discussion: https://postgr.es/m/17564-c7472c2f90ef2da3@postgresql.org

src/test/regress/sql/select_parallel.sql

@@ -435,6 +435,12 @@ EXPLAIN (VERBOSE, COSTS OFF)
 SELECT generate_series(1, two), array(select generate_series(1, two))
   FROM tenk1 ORDER BY tenthous;
 
+-- must disallow pushing sort below gather when pathkey contains an SRF
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT unnest(ARRAY[]::integer[]) + 1 AS pathkey
+  FROM tenk1 t1 JOIN tenk1 t2 ON TRUE
+  ORDER BY pathkey;
+
 -- test passing expanded-value representations to workers
 CREATE FUNCTION make_some_array(int,int) returns int[] as
 $$declare x int[];