Add unit tests for iis7_add_rewrite_rule()

[pbearne]

Description:
This PR adds unit tests for the iis7_add_rewrite_rule() function in wp-admin/includes/misc.php. These tests ensure that WordPress rewrite rules are correctly added to IIS7 web.config files, handling cases where configuration elements might be missing.

The tests cover:

• Adding a rule when the file does not exist.
• Adding a rule to an empty or partial configuration.
• Ensuring rules are not duplicated if they already exist (case-insensitive).
• Correct behavior with existing other rules.
• Handling of invalid XML.

Trac ticket: https://core.trac.wordpress.org/ticket/65172

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[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.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @pbearne@git.wordpress.org.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.