Fix logic on how PHPUnit for WP is fetched
Fix logic on how PHPUnit for WP is fetched

Always downloading the latest PHPUnit from the Wordpress.org SVN resulted in incombatible tests being run.

For example, in the nightly version that comes after the release 5.4.2, PHPMailer was renamed from `class-phpmailer.php` to `PHPMailer.php`.
Due to this, the tests try to run the file only present in the nightly release.

Now fetch the tests from WordPress.org GitHub mirror and use the correct version number.

Seravo/wordpress-test-template,Seravo/wordpress-test-template,Seravo/wordpress-test-template