Build/Test Tools: Split the PHPUnit tests for PHP versions below 7.0 in half, allowing them to run in parallel and reduce the overall test run duration on GitHub Actions.

The `external-http`, `media`, and `restapi` groups combined take roughly the same time as all the other tests combined. Splitting the test run in two speeds up the PHP 5.6 test runs which are the slowest by quite a margin. The net reduction in duration is only seen on PHP versions older than 7.0 where the reduction is greater than the time taken for the job to be set up.

Props peterwilsoncc, desrosj, johnbillion

Special shout out to manzoorwanijk whose article about running steps in parallel started me down this path.

Fixes #52645

Built from https://develop.svn.wordpress.org/trunk@50444


git-svn-id: http://core.svn.wordpress.org/trunk@50055 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50443';
+$wp_version = '5.8-alpha-50444';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.