Header Test Document Refactor (#3168)

* refactors header testing document to For/Given/When/Then structure and separates out testing examples

* code display changes and moves a singular use example to its own test

* spelling correction

Co-authored-by: rmccar <[email protected]>

* typo correction

Co-authored-by: rmccar <[email protected]>

* correct capitalisation

Co-authored-by: rmccar <[email protected]>

* updates code in line with comments

* add correct breakpoint

* updates multiple link test to check for relevant classes

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Co-authored-by: Alessio Venturini <[email protected]>
Co-authored-by: rmccar <[email protected]>