Addition of --no-sync to pg_upgrade for test speedup

Hi all,

As per $subject, avoiding the flush of the new cluster's data
directory shortens a bint the runtime of the test.  In some of my slow
VMs, aka Windows, this shaves a couple of seconds even if the bulk of
the time is still spent on the main regression test suite.

In pg_upgrade, we let the flush happen with initdb --sync-only, based
on the binary path of the new cluster, so I think that we are not
going to miss any test coverage by skipping that.

Thoughts or opinions?
--
Michael