[Storefront API Client] Reorganize tests for server and browser testing

[melissaluu]

WHY are these changes introduced?

Reorganize the tests to clearly state tests specific for either server or browser environment.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used yarn changeset to create a draft changelog entry (do NOT update the CHANGELOG.md file manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[paulomarg]

Love it! Didn't really go into the tests because I'm assuming we're just moving things around, right?

[paulomarg]

Oh I really like the concept of a fixtures file. Makes it much easier to find stuff!

[melissaluu]

I'm assuming we're just moving things around, right?

Yup! I didn't add or remove anything - it's just reorganizing things...