Fix thinko with subdirectories generated by pg_upgrade for internal files

Fix thinko with subdirectories generated by pg_upgrade for internal files

38bfae3 has mixed the "dump/" and "log/" subdirectories generated in
"pg_upgrade_output.d/", causing the internal dump files to be generated
in "log/" and the log files to be in "dump/", but the opposite should be
done.  This was not directly an issue for pg_upgrade runs, as the
internal dump files were still picked up at the location of their
creation, but the newest version of the buildfarm client would have
reported the dump files instead of the log files on failures of
pg_upgrade.

Issue spotted while testing the TAP tests of pg_upgrade.