Provide Apex unit tests for the Apex used by the LWC reference components

Currently https://github.com/forcedotcom/b2b-commerce-on-lightning-quickstart/tree/master/examples/lwc/force-app/main/default/classes has no unit tests.

With the recommended pattern being to copy the reference component code as a starting point when customization is needed, it would be useful for the Apex unit tests to be included as a starting point to be copied.

Start by reviewing the Apex classes in examples/lwc/force-app/main/default/classes and the related LWC reference components. Add unit tests for the Apex used by those components, with coverage sufficient for the reference behavior, then run the repository's Apex test suite to verify the tests pass.

Written by the indexing model from the issue text.