Enhancement: Allow to overwrite `WP_UnitTestCase_Base::factory()` method.

[hugod]

What?

This is a enhancement request in order to overwrite WP_UnitTestCase_Base::factory() static method in child classes of WP_UnitTestCase_Base.

Why?

To be able to overwrite WP_UnitTest_Factory.
As an example, as third party plugin could write its own factory extending WP_UnitTest_Factory for testing purposes.
And to benefit from WP_UnitTestCase_Base features, its test case would also extend it.
But overwriting WP_UnitTestCase_Base::factory() has no effect since it's called with the self keyword instead of the static keyword which has late static binding (see ​here and ​here).

## What?
Remove self keyword in favor of static in WP_UnitTestCase_Base so factory() can be easily ovewriten.

Trac ticket: #59999

[SergeyBiryukov]

In 57149:

Build/Test Tools: Allow overriding the WP_UnitTestCase_Base::factory() method.

This allows third-party plugins to write their own factory extending WP_UnitTest_Factory for testing purposes, as well as benefit from WP_UnitTestCase_Base features.

Follow-up to [35186], [35225], [35242].

Props hugod.
Fixes #59999.

Thanks for the PR! Merged in r57149.