This updates the fstest implementation of a storage.FS to support
opening sub containers and files in read-only mode.
Now when calling `SubContainer`:
- If using `storage.WithCreateFile`, an error is returned if the
container already exists.
- If not using `storage.WithCreateFile` and error is returned if the
container does *not* exist.
When calling `OpenFile`:
- on a container that was created with `storage.ReadOnly`, an error is
returned if attempting to open the file wth `storage.ReadWrite`.
- with `storage.WithCreateFile`, an existing file is replaced with a new
empty file, (ie "truncated").
- without `storage.WithCreateFile`, an error is returned if the file
does not exist.
Timothy Messier
46e1752283