The 'Clear Inverse' operator didn't properly update the constraint, so
it didn't do anything until the entire depsgraph was updated. It's now
properly tagged for update.
In the collections unit test file developers can now disable layer
collections and declutter the 3D Viewport while working in
`constraints.blend`, without influencing the actual unit tests themselves.
Currently this only tests the Child Of constraint. My aim is to cover
constraints with tests before they are refactored/altered.
No functional changes.
