mgr/nfs: add additional tests for cmount_path & user_id deletion

mgr/nfs: add additional tests for cmount_path & user_id deletion

Add unit tests for unique user ID generation, deletion and `cmount_path` handling in FSAL exports

- Ensure unique user ID generation for different FSAL blocks when creating exports.
- Test deletion behavior when multiple exports share the same user ID and one has a unique ID.
- Test default behavior when no `cmount_path` is provided (defaults to `/`).
- Add tests to validate error handling for invalid `cmount_path` values.

Signed-off-by: Avan Thakkar <athakkar@redhat.com>