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