Build/Test Tools: Make the polyfills loading more flexible.

The PHPUnit Polyfills are, since [51559], a required dependency for the WP test suite and, by extension, for plugin/theme integration test suites which are based on and use (parts of) the WP core test suite.

However, plugin/theme integration test suites may not use a full WordPress installation.

This commit:
* Removes the presumption that a full WP install, including `vendor` directory, will be available when the test `bootstrap.php` file is run.
* Makes the loading of the PHPUnit Polyfills autoload file more flexible by:
  * Checking if the autoload class contained within the autoload file is already available before attempting to load the file.[[BR]]
    This allows for plugin/theme integration test suites to load the `phpunitpolyfills-autoload.php` file from any location, as long as it is loaded before the WP core test `bootstrap.php` file is run.
  * Allowing for the path to an arbitrary installation location for the PHPUnit Polyfills to be passed as a constant.[[BR]]
    As long as the provided location is a valid file path and the `phpunitpolyfills-autoload.php` file exists in the provided location, that file will be loaded.[[BR]]
    The constant can be declared in a plugin/theme integration test suite native test bootstrap file, or in the `wp-tests-config.php` file, or even in a `phpunit.xml[.dist]` file via `<php><const name="WP_TESTS_PHPUNIT_POLYFILLS_PATH" value="path/to/yoast/phpunit-polyfills"/></php>`.
* Adds a version check for the PHPUnit Polyfills to prevent a mismatch between the version of the package expected by WordPress and the version used by plugins/themes.[[BR]]
  The version this checks for should be in line with the minimum version requirement for the PHPUnit Polyfills as declared in the `composer.json` file.[[BR]]
  This version number should only be updated when new features added in later PHPUnit Polyfills releases are actually used in the WP core test suite.
* Adds appropriate error messages for every possible error condition.
* Upgrades the PHPUnit Polyfills to version 1.0.1, which now includes a version constant.

Follow-up to [51559-51577].

Props jrf, hellofromTonya, swissspidy, jeherve, thomasplevy, SergeyBiryukov.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51598


git-svn-id: http://core.svn.wordpress.org/trunk@51209 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51597';
+$wp_version = '5.9-alpha-51598';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.