Expose CartTestProviders in public API

[LewisN]

Description

Mock providers were exposed to a public API in PR #2224 but CartTestProviders was not included.

This PR includes CartTestProviders in the public mock providers so it can be used to provide Cart Context in external contexts (e.g. Storybook).

Fixes issue #2323

Usage in Storybook:

import { ComponentMeta, ComponentStory } from '@storybook/react'
import React from 'react'
import ComponentName from './ComponentName.ui'
import { ShopifyTestProviders, CartTestProviders } from '@shopify/hydrogen/testing'

export default {
  title: 'Components/ComponentName',
  component: ComponentName,
  decorators: [],
} as ComponentMeta<typeof ComponentNameUI>

const Template: ComponentStory<typeof ComponentNameUI> = (args) => {
  return (
    <ShopifyTestProviders>
      <CartTestProviders>
        <Component {...args} /> // This component imports import { useCart } from '@shopify/hydrogen'
      </CartTestProviders>
    </ShopifyTestProviders>
  )
}

export const ComponentName = Template.bind({})

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• [x] Read the Contributing Guidelines
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123)
• [ ] Update docs in this repository according to your change
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