Use len instead of for-loop to get the end index

Use len() instead of for-loop to get the end index in file
cinder/tests/test_backup_tsm.py

Change-Id: I3a7b0e418f8af4881e21eb5273a03dbcd317c27d
Closes-Bug: #1282084
Merge "Use len instead of for-loop to get the end index"