Allow unit tests to override is_*_admin()

[ryan]

Unit tests sometimes need to fake being in the user, network, or blog admin or on the front page. This is currently impossible due to the use of constants. A possible solution is to consult the current screen if it is setup and fallback to the constants otherwise. Alternatively, add filters in the is_*_admin() functions.

Rough take on using WP_Screen

In [21687]:

Introduce WP_Screen::in_admin() for determining which admin the screen is in.
Change is_*_admin() to reference in_admin() with fallback to the WP_*_ADMIN constants during early bootstrap. This allows unit tests and ajax handlers to set the admin context.

fixes #21742

Missing dashboard widgets: ​http://cl.ly/J9gm

In [21693]:

s/blog/site/. see #21742