Build/Test Tools: Unify the PHPUnit adapter TestCases.

This commit:
* Removes the PHPUnit 7 specific `TestCase`.
* Removes all existing polyfills from the PHPUnit 5.x `TestCase`.
* Imports all polyfill traits from the PHPUnit Polyfills package into the `WP_UnitTestCase` class and updates the DocBlock to reflect the actual function of the class.
 * Note: The list of polyfills needs to be verified and updated after each new release of the PHPUnit Polyfills package. Alternatively (recommended), one of the built-in `TestCase` classes from the PHPUnit Polyfills package can be used instead.

* Moves the `require` for the WP `abstract-testcase.php` to the `bootstrap.php` file.
* Adds a `require_once` for the PHPUnit Polyfills autoloader to the `bootstrap.php` file.
 * Note: while this isn't _strictly_ necessary when the tests are run via Composer, having the include in the bootstrap allows for the tests to also be run via a PHPUnit Phar, providing contributors with more flexibility.

Follow-up to [51559].

Props jrf, hellofromTonya, johnbillion, netweb, SergeyBiryukov.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51560


git-svn-id: http://core.svn.wordpress.org/trunk@51171 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51559';
+$wp_version = '5.9-alpha-51560';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.