Build/Test Tools: Alias the Getopt class conditionally, as the class no longer exists in PHPUnit 9.x.

Most of the aliasing in this `compat.php` file is redundant as PHPUnit 5.7.21+ contains a forward compatibility layer for these classes anyway (= PHPUnit provides both the namespaced and underscore named versions of these classes in PHPUnit 5.7.21+).

All the same, the file and the aliases are left in place for the time being, as plugins/themes using the WP test suite as the basis for their integration tests may rely on it, though WP itself should not really need it anymore, save for maybe one or two classes.

Follow-up to [51559-51569].

Props jrf.
See #46149.

git-svn-id: https://develop.svn.wordpress.org/trunk@51570 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tests/phpunit/includes/phpunit6/compat.php

@@ -14,7 +14,9 @@ class_alias( 'PHPUnit\Framework\AssertionFailedError', 'PHPUnit_Framework_Assert
 	class_alias( 'PHPUnit\Framework\TestSuite', 'PHPUnit_Framework_TestSuite' );
 	class_alias( 'PHPUnit\Framework\TestListener', 'PHPUnit_Framework_TestListener' );
 	class_alias( 'PHPUnit\Util\GlobalState', 'PHPUnit_Util_GlobalState' );
-	class_alias( 'PHPUnit\Util\Getopt', 'PHPUnit_Util_Getopt' );
+	if ( class_exists( 'PHPUnit\Util\Getopt' ) ) {
+		class_alias( 'PHPUnit\Util\Getopt', 'PHPUnit_Util_Getopt' );
+	}
 
 	class PHPUnit_Util_Test {