Add unit tests for `core/modules/catalog`

[pkarw]

What is the motivation for adding/enhancing this feature?

Add unit tests for the module. Reference/example:

• https://github.com/DivanteLtd/vue-storefront/tree/develop/core/modules/catalog-next/test/unit
• https://github.com/DivanteLtd/vue-storefront/blob/develop/core/modules/url/test/unit/formatCategoryLink.spec.ts

What are the acceptance criteria

• [ ] all the actions should be tested
• [ ] all the helpers should be tested
• [ ] all the mutations should be tested

[lukeromanowicz]

@pkarw not sure if there is much point in writing tests for the old catalog. It's a legacy module anyway.

[pkarw]

It’s still being used by Product.vue page and nothing shows it’s going to disappear in foreseeable future :)