Build/Test Tools: Loosen the PHPUnit restriction.

**composer.json**:

Remove the PHPUnit dependency in favor of allowing the PHPUnit Polyfills library to manage the supported PHPUnit version. This automatically now widens the supported PHPUnit versions to 5.7.21 to 9.5.8 (current).

Letting the PHPUnit Polyfills handle the version constraints for PHPUnit prevents potential version conflicts in the future, as well as allows WordPress to benefit straight away when a new PHPUnit version would be released and the PHPUnit Polyfills package adds support for that PHPUnit version.

**Test Bootstrap**

Update the supported version number for PHPUnit 5.x, as the minimum PHPUnit 5.x version supported by the PHPUnit Polyfills is PHPUnit 5.7.21, and remove the PHPUnit maximum.

**.gitignore and svn:ignore:**

Add the PHPUnit cache file to the list of files to be ignored.

Since PHPUnit 8, PHPUnit has a built-in caching feature which creates a `.phpunit.result.cache` file. This file should not be committed.

Follow-up to [40536], [40853], [44701], [51559-51573].

Props jrf.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51574


git-svn-id: http://core.svn.wordpress.org/trunk@51185 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51573';
+$wp_version = '5.9-alpha-51574';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.