Build/Test Tools: Change the inheritance order of the abstract test classes.

As things were, the inheritance order of the abstract test classes was as follows:
{{{
WP_UnitTestCase (PHPUnit adapter layer)
    extends WP_UnitTestCase_Base (base test class)
        extends PHPUnit\Framework\TestCase (PHPUnit native class)
}}}

Concrete (child) test classes, as well as more specific abstract TestCases, are/were expected to extend the `WP_UnitTestCase`.

This order is not optimal as it means that the `WP_UnitTestCase_Base` class would not be able to benefit from any polyfills and/or shims in the PHPUnit adapter layer.

With that in mind, this commit changes the inheritance to:
{{{
WP_UnitTestCase (empty class, left in place to not break BC for plugin/theme integration tests)
    extends WP_UnitTestCase_Base (base test class)
        extends PHPUnit_Adapter_TestCase (PHPUnit adapter layer)
            extends PHPUnit\Framework\TestCase (PHPUnit native class)
}}}

The new order allows for the `WP_UnitTestCase_Base` to also benefit from the PHPUnit adapter layer.

For backward compatibility reasons the `WP_UnitTestCase`, which all test classes are (were) expected to extend, is left in place, though it is now an empty class and explicitly `abstract`.

Follow-up to [51559], [51560].

Props jrf, hellofromTonya, johnbillion, netweb, SergeyBiryukov.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51561


git-svn-id: http://core.svn.wordpress.org/trunk@51172 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51560';
+$wp_version = '5.9-alpha-51561';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.