MB-100 correct the backup test in sanity conf file

Change-Id: I66951182036c1b51135b9a8072071e40aa7342cd
Reviewed-on: http://review.couchbase.org/25732
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>
Tested-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>