Upgrade the integration tests for the cart module

[pkarw]

What is the motivation for adding / enhancing this feature?

After the #2887 the cart module got refactored, we need to update our unit tests to cover all the new interfaces

What are the acceptance criteria

• the unit tests are extended to cover all the public actions in the cart module (https://github.com/DivanteLtd/vue-storefront/blob/feature/2513/core/modules/cart/store/actions.ts)

Can you complete this feature request by yourself?

• YES
• NO

Which Release Cycle state this refers to? Info for developer.

Pick one option.

• This is a normal feature request. This should be available on https://test.storefrontcloud.io and then after tests this can be added to next Vue Storefront version. In this case Developer should create branch from develop branch and create Pull Request 2. Feature / Improvement back to develop.
• (Pick this option only if you're sure) This is an important improvement request for current Release Candidate version on https://next.storefrontcloud.io and should be placed in next RC version. In this case Developer should create branch from release branch and create Pull Request 3. Stabilisation fix back to release.
• (Pick this option only if you're sure) This is a critical improvement request for current Stable version on https://demo.storefrontcloud.io and should be placed in next stable version. In this case Developer should create branch from hotfix or master branch and create Pull Request 4. Hotfix back to hotfix.

Additional information

[pkarw]

@andrzejewsky Patryk, before you'll start the checkout refactor let's have cart tests finished - we're after all these refactorings so it would be great to finish that one first