Typo corrections for test files in cinder

Correct the typo from follow code files:

cinder/tests/api/contrib/test_backups.py
cinder/tests/test_migrations.py
cinder/tests/test_netapp.py

Change-Id: I6aeddfee36ecd101264d73d2e90686df683cc29d
Merge "Typo corrections for test files in cinder"