This changes 020_cancel.pl so as the test is entirely skipped on
Windows. This test was already doing nothing under WIN32, except
initializing and starting a node without using it so this shaves a few
test cycles.
Author: Yugo NAGATA
Reviewed-by: Fabien Coelho
Discussion: https://postgr.es/m/20230810125935.22c2922ea5250ba79358965b@sraoss.co.jp
Backpatch-through: 15