Tests: Add REST API user creation email notification tests.

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/40477

This PR adds unit tests verifying email notifications are sent when creating users through the REST API. Tests cover:

• Default admin notification behavior
• Filter override capability via rest_wp_user_created_notification
• Multi-recipient notifications when using 'both' parameter

Follows the implementation from #8572 by @nikunj8866.

[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 abcd95, johnjamesjacoby, westonruter.

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

• The Plugin and Theme Directories cannot be accessed within Playground.
• 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.

[himanshupathak95]

The tests fail only in the multisite tests since, in those environments, we also need Super Admin privileges to create users. I will update the tests accordingly.

[JJJ]

See comments...

[himanshupathak95]

I don't see why the performance test must fail here, definitely unrelated to the changes in the PR.