Standardize CBMC stub filenames

At present, there is no naming convention for stub filenames: Some of them have as suffix `stub` while others use `overrides` or something else. We should come up with a standard for naming these stubs and apply it here.

No files, tests, or entry points are named. Start by inventorying the existing CBMC stub filenames and the places that reference them, then agree on a naming convention before applying it. Done means the convention is decided and all affected stub names and references consistently follow it.

Written by the indexing model from the issue text.