Add test for pg_upgrade file transfer modes.

Add test for pg_upgrade file transfer modes.

This new test checks all of pg_upgrade's file transfer modes.  For
each mode, we verify that pg_upgrade either succeeds (and some test
objects successfully reach the new version) or fails with an error
that indicates the mode is not supported on the current platform.
For cross-version tests, we also check that pg_upgrade transfers
non-default tablespaces.  (Tablespaces can't be tested on same
version upgrades because of the version-specific subdirectory
conflict, but we might be able to enable such tests once we teach
pg_upgrade how to handle in-place tablespaces.)

Suggested-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/Zyvop-LxLXBLrZil%40nathan