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.

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[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.