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.

git-svn-id: https://develop.svn.wordpress.org/trunk@51561 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/bootstrap.php

@@ -207,6 +207,7 @@ if ( version_compare( tests_get_phpunit_version(), '6.0', '>=' ) ) {
 require_once $phpunit_polyfills_autoloader;
 unset( $phpunit_polyfills_autoloader );
 
+require __DIR__ . '/phpunit-adapter-testcase.php';
 require __DIR__ . '/abstract-testcase.php';
 require __DIR__ . '/testcase.php';
 require __DIR__ . '/testcase-rest-api.php';