Add PHPUnit tests for links_add_base_url function

[pbearne]

A new PHPUnit test class, Tests_formatting_linksAddBaseUrl, has been introduced to increase the test coverage for the links_add_base_url function. The class includes multiple test scenarios, examining different combinations of attributes to verify the correct functioning of the function.

Trac ticket: https://core.trac.wordpress.org/ticket/60389

[github-actions]

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[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props pbearne, peterwilsoncc, jorbin, mukesh27, aristath.

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[peterwilsoncc]

Naming things and a realtive scheme test.

Please confirm any coding stanadards and results.

[mukeshpanchal27]

@pbearne Needs to fix the PHPCS fix.

[pbearne]

@pbearne Needs to fix the PHPCS fix.

fixed

[aristath]

LGTM 👍

[peterwilsoncc]

Merged r59162 / 636d748