Validate spread expressions before packing in `*craft test`

### What needs to get done

Before running the packing step, any spread expressions passed to the command.

### Why it needs to get done

Saves user time

Start at the `*craft test` command and trace how spread expressions are handled relative to the packing step. Done means spread expressions are validated before packing begins, with the validation outcome covered by the command's existing tests if their location can be found.

Written by the indexing model from the issue text.