pgsql: Put back regression test case in a more robust form.

Put back regression test case in a more robust form.

This undoes my hurried commit 776a2c887, 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/22864f6e02f8fc19f6167442f13d1f917e36548e

Modified Files
--------------
src/test/regress/expected/partition_prune.out | 13 +++++++++++++
src/test/regress/sql/partition_prune.sql      |  7 +++++++
2 files changed, 20 insertions(+)