MB-100: close RemShellCon in basetestcase

Change-Id: Ib59c340c5135accbc9fb4b944794b6ec1e80902f
Reviewed-on: http://review.couchbase.org/25308
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>