MB-100: run tearDown in setup if it failed in prev test

Change-Id: I414a3fdc3946d47199a0454f3381c3b24c44ee4d
Reviewed-on: http://review.couchbase.org/24835
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>