Build/Test Tools: Allow unit tests to run on PHP 8 in full.

PHP 8 introduces a new control structure called `match`, which makes `match` a reserved keyword in PHP 8.

One of the PHPUnit dependencies declares a class named `Match`, which triggered a fatal error before PHPUnit could even start.

To be able to use PHPUnit 7.x on PHP 8 and run the tests, core needs a new version of that dependency, which is now installed using Composer.

This is the simplest way to get things working again and start addressing the individual test failures.

Additionally, various test runs on PHP 8 on Travis are now performed individually instead of being chained, so that failures outside of WP scope don't block further execution.

Props jrf, jorbin, pento.
See #50902.
Built from https://develop.svn.wordpress.org/trunk@48957


git-svn-id: http://core.svn.wordpress.org/trunk@48719 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-48954';
+$wp_version = '5.6-alpha-48957';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.