CBQE-1003: minor fix in tearDown of xdcrupgrade tests

Change-Id: I24ff4a4c294373b9f9f465893456d984db4ef949
Reviewed-on: http://review.couchbase.org/25008
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>