pgsql: Attempt to stabilize partition_prune test output (2)

Attempt to stabilize partition_prune test output (2)

Environmental conditions might cause parallel workers to be scheduled in
different ways in this test, destabilizing the EXPLAIN output.  Disable
use of workers in an attempt to make output stable.

Author: David Rowley
Diagnosed-by: Thomas Munro
Discussion: https://postgr.es/m/CAKJS1f8j24tUX_nOwACiM=UO5jrMrDz8ca0xbG0vhVgfWph0ZA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4d0f6d3f207d977d3685499263993206b56d425a

Modified Files
--------------
src/test/regress/expected/partition_prune.out | 79 ++++++++++++++-------------
src/test/regress/sql/partition_prune.sql      |  5 ++
2 files changed, 46 insertions(+), 38 deletions(-)