Rather than assuming that the storage instance will be attached to a VM,
just return it. Callers may attach it to a `VM` or (in the case of
`import_sb2.js`) to a `Runtime`.
- Attach the storage module to the VM for tests which load projects or
other assets.
- Move `import_sb2.js` from `test/unit/ into `test/integration` since it
now depends heavily on `scratch-storage`.
- Skip loading costumes when there is no renderer.
