[BUG] CartTestProviders is not available for testing

[LewisN]

Describe the bug
Components that rely on a Cart Context cannot be rendered in Storybook. This error is shown:

Error: Expected a Cart Context, but no Cart Context was found

CartTestProviders already exists as a mock Provider which could resolve this, but it isn't exported by @shopify/hydrogen/testing.

To Reproduce

1. Add Storybook to a Hydrogen project (v1.5.0 and above)
2. Create a story rendering the Header.client component from the Hydrogen demo store (or any component that uses the useCart hook and relies on a Cart Context)
3. Run storybook and try to load the story

Expected behaviour
I should be able to import CartTestProviders from @shopify/hydrogen/testing and use this in a decorator to provide Cart Context (the same way I can with ShopifyTestProviders). Component should render in Storybook without errors.

Screenshots

Additional context

• Hydrogen version: 1.5.0+