Restore compatibility with WordPress <6.1

[BrianHenryIE]

WordPress 6.1 renamed wp-includes/wp-db.php to wp-includes/class-wpdb.php, WP Browser looks only for wp-includes/class-wpdb.php. This PR checks does it exist and otherwise uses the older file. This allows testing older versions of WordPress.

$ codecept run wpunit

Codeception PHP Testing Framework v4.2.2 https://helpukrainewin.org
Powered by PHPUnit 9.6.20 by Sebastian Bergmann and contributors.
PHP Warning:  require_once(/Users/brian.henry/Sites/my-plugin/wordpress/wp-includes/class-wpdb.php): Failed to open stream: No such file or directory in /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/includes/core-phpunit/includes/install.php on line 43
PHP Stack trace:
PHP   1. {main}() /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/includes/core-phpunit/includes/install.php:0
PHP Fatal error:  Uncaught Error: Failed opening required '/Users/brian.henry/Sites/my-plugin/wordpress/wp-includes/class-wpdb.php' (include_path='.:/opt/homebrew/Cellar/php@8.0/8.0.30_3/share/php@8.0/pear') in /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/includes/core-phpunit/includes/install.php:43
Stack trace:
#0 {main}
  thrown in /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/includes/core-phpunit/includes/install.php on line 43
PHP Fatal error:  Uncaught Codeception\Exception\ModuleException: lucatume\WPBrowser\Module\WPLoader: WordPress bootstrap failed.
 in /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/src/Module/WPLoader.php:1033
Stack trace:
#0 [internal function]: lucatume\WPBrowser\Module\WPLoader->lucatume\WPBrowser\Module\{closure}('', 8)
#1 {main}
  thrown in /Users/brian.henry/Sites/my-plugin/vendor/lucatume/wp-browser/src/Module/WPLoader.php on line 1033
Script codecept run wpunit handling the test event returned with error code 255

• https://core.trac.wordpress.org/ticket/56268
• WordPress/WordPress@8484c7b

[lucatume]

Thanks @BrianHenryIE for the report and the fix.

That file is pulled form the Core PHPUnit test suite by means of the bin/update_core_phpunit_includes script. To keep that process automated, I will need to update the patches to included the install.php file.
This is to say the fix is less straighforward that it might look.

I will build on this to include the fix in the automation flow.