Add tests for consistency groups DB migration

The consistency group patch added 2 new DB migration versions:
025 and 026. However, there were no unit tests to support them in
cinder/tests/test_migrations.py. This patch added missing tests.

Change-Id: I670825980fddd4b6dc400e5a17e1003ad47ace1a
Closes-Bug: #1370756
Merge "Add tests for consistency groups DB migration"