prevent 006_transfer_modes.pl from leaving files behind

The other pg_upgrade tests chdir to tmp_check prior to running pg_upgrade
to avoid leaving behind delete_old_cluster.{sh,bat}. 006_transfer_modes.pl
should, too. However, this test is a little different because it loops
over all of the available transfer modes and runs pg_upgrade for each one
supported by the platform. From my testing and code analysis, it seems
sufficient to change the directory once at the beginning of the test, but
we could alternatively save the current directory and change back to it in
each iteration to be safe.

Thoughts?

--
nathan