Build/Test Tools: Implement use of the void solution.

> PHPUnit 8.0.0 introduced a `void` return type declaration to the "fixture" methods – `setUpBeforeClass()`, `setUp()`, `tearDown()` and `tearDownAfterClass()`. As the `void` return type was not introduced until PHP 7.1, this makes it more difficult to create cross-version compatible tests when using fixtures, due to signature mismatches.
>
> The `Yoast\PHPUnitPolyfills\TestCases\TestCase` overcomes the signature mismatch by having two versions. The correct one will be loaded depending on the PHPUnit version being used.
>
> When using this TestCase, if an individual test, or another TestCase which extends this TestCase, needs to overload any of the "fixture" methods, it should do so by using a snake_case variant of the original fixture method name, i.e. `set_up_before_class()`, `set_up()`, `assert_pre_conditions()`, `assert_post_conditions()`, `tear_down()`, and `tear_down_after_class()`.
>
> The snake_case methods will automatically be called by PHPUnit.
>
> > IMPORTANT: The snake_case methods should not call the PHPUnit parent, i.e. do not use `parent::setUp()` from within an overloaded `set_up()` method. If necessary, DO call `parent::set_up()`.

Reference: https://github.com/Yoast/PHPUnit-Polyfills#testcases

This commit renames all declared fixture methods, and calls to parent versions of those fixture methods, from camelCase to snake_case.

Follow-up to [51559-51567].

Props jrf, hellofromTonya, johnbillion, netweb, dd32, pputzer, SergeyBiryukov.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51568


git-svn-id: http://core.svn.wordpress.org/trunk@51179 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51567';
+$wp_version = '5.9-alpha-51568';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.