Footer credit: Add unit tests

[mmtr]

Follow-up of #38559

Proposed changes:

Adds a new unit test suite that checks the expected behavior of the Footer Credit feature in the Customizer in order to avoid the particular failure that caused the revert in #38557.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

N/A

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Go to '..'

[github-actions]

Thank you for your PR!

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[github-actions]

This PR has been marked as stale. This happened because:

• It has been inactive for the past 3 months.
• It hasn’t been labeled `[Pri] BLOCKER`, `[Pri] High`, etc.

If this PR is still useful, please do a [trunk merge or rebase](https://github.com/Automattic/jetpack/blob/trunk/docs/git-workflow.md#keeping-your-branch-up-to-date) and otherwise make sure it's up to date and has clear testing instructions. You may also want to ping possible reviewers in case they've forgotten about it. Please close this PR if you think it's not valid anymore — if you do, please add a brief explanation.

If the PR is not updated (or at least commented on) in another month, it will be automatically closed.