Improve code coverage for pkg/common-controller

[ggriffiths]

Need to improve code coverage after the split controller refactor.
/assign @ggriffiths
/cc @xing-yang
/kind cleanup

[ggriffiths]

Still need to revisit and update the following:

• pkg/common-controller/snapshot_create_test.go (a few need to be re-visited)
• pkg/common-controller/snapshot_delete_test.go (most need to be re-written)
• pkg/common-controller/snapshot_update_test.go (a few need to be re-visited)