Introduce a filter for `self_admin_url()`

[flixos90]

I'm currently building a plugin which includes an additional administration panel (like the network or user admin). However, the function self_admin_url() is impossible to adjust to that new functionality. I would like to be able to have it return the admin URL to my new administration panel if it is active.

Therefore I propose to introduce a new filter self_admin_url for the function. A developer could hook into that, check whether the custom administration panel is currently active and then adjust the returned URL accordingly.

[flixos90]

37446.diff​ introduces the filter.

[johnjamesjacoby]

+1

[DrewAPicture]

+1. We should probably also pass the $scheme value to the filter for completeness.

[flixos90]

Related: #37526

[j.hoffmann]

In 37446.2.diff​ I added the $scheme parameter.

[flixos90]

In 37446.3.diff​ I added a few unit tests for self_admin_url().

It's currently not possible to test the function further without hacking a lot, since there is no way to influence function outcomes from e.g. is_network_admin(), since WP_Screen is final and cannot be mocked. The few tests that are there now at least perform the most basic checks and ensure the filter works.

[flixos90]

In 41060:

Multisite: Introduce a self_admin_url filter to adjust the URL to an administration panel.

Props j.hoffmann.
Fixes #37446.