Header Test Document Refactor

What is the context of this PR?

Follows the investigation and proposal done for #2822, making alterations as necessary to the proposed structure in response to our specific testing requirements.

Fixes: #3120

The conclusion of this ticket will spawn tickets for the following:

Fully documenting the new test doc structure: clear approach and important info for refactoring other test docs/writing new tests.
Refactoring all remaining components (each component will be a separate ticket).

How to review this PR

Quite a few things to think about with this one!

Functionality:

Run the test suite using: yarn test --testNamePattern="FOR: Header"

Does this testing refactor meet the following success criteria:

Clearly structured.
Follows naming conventions.
Consistent.
Concise.
Suited to our codebase.
Compatible with existing testing framework.

Look at the refactored code in relation to:

The original test document.
The macro file.
The macro options file.

Other Considerations (discussed in tech session 30/04):

Multiple set-ups - double check you're happy with the layout for this.
Consistent testing - see if there's any tests that you think should be further split out or condensed into one!

Checklist

This needs to be completed by the person raising the PR.

[x] I have selected the correct Assignee
[x] I have linked the correct Issue

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ONSdigital / design-system