pgsql: Add proper regression test for the recent SRFs-in-pathkeys probl

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cc116479917b7768bef46d0d789093bd6b25f7cd

Modified Files
--------------
src/test/regress/expected/incremental_sort.out | 12 ------------
src/test/regress/expected/select_parallel.out  | 24 ++++++++++++++++++++++++
src/test/regress/sql/incremental_sort.sql      |  2 --
src/test/regress/sql/select_parallel.sql       |  6 ++++++
4 files changed, 30 insertions(+), 14 deletions(-)