Add option -N/--no-sync to pg_upgrade

Add option -N/--no-sync to pg_upgrade

This is an option consistent with what the other tools of src/bin/
(pg_checksums, pg_dump, pg_rewind and pg_basebackup) provide which is
useful for leveraging the I/O effort when testing things.  This is not
to be used in a production environment.

All the regression tests of pg_upgrade are updated to use this new
option.  This happens to cut at most a couple of seconds in environments
constrained on I/O, by avoiding a flush of data folder for the new
cluster upgraded.

Author: Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/YbrhzuBmBxS/DkfX@paquier.xyz