Unit tests should typically not use `wp_delete_user()` to delete users for multisite compatibility

[flixos90]

Several PHPUnit tests rely on wp_delete_user() to delete temporarily created users. However, when using WordPress Multisite, wpmu_delete_user() needs to be used instead. For this purpose, the WordPress test base class provides a static method delete_user( $user_id ), which abstracts away this detail.

All PHPUnit tests should use this static method instead of wp_delete_user(), unless the test is explicitly excluding multisite or testing a behavior of wp_delete_user() with consideration of how it behaves differently with a multisite network.

Trac ticket: https://core.trac.wordpress.org/ticket/61851

[flixos90]

In 58876:

Build/Test Tools: Avoid using wp_delete_user() in PHPUnit tests unless explicitly acknowledging or ignoring Multisite.

wp_delete_user() does not actually delete the entire user when using WordPress Multisite. Therefore tests should typically use the test helper method to fully delete the user, unless explicitly ignoring Multisite or testing the wp_delete_user() function while taking Multisite behavior into account.

Fixes #61851.

Committed in https://core.trac.wordpress.org/changeset/58876