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