Make sure PHPUnit_Framework_TestCase doesn't already exist.

Author: octalmage

tests/phpunit/bootstrap.php

@@ -21,6 +21,7 @@
 	$test_root = '../../../../../../../tests/phpunit';
 }
 
+// WordPress versions before 4.8 will be incompatible with newer PHPUnit versions.
 if ( version_compare( getenv( 'WP_VERSION' ), '4.8', '<' ) && class_exists( 'PHPUnit\Runner\Version' ) ) {
 	require_once dirname( __FILE__ ) . '/phpunit6-compat.php';
 }

tests/phpunit/phpunit6-compat.php

@@ -1,5 +1,8 @@
 <?php
-if ( class_exists( 'PHPUnit\Runner\Version' ) && version_compare( PHPUnit\Runner\Version::id(), '6.0', '>=' ) ) {
+if (
+	class_exists( 'PHPUnit\Runner\Version' ) &&
+	version_compare( PHPUnit\Runner\Version::id(), '6.0', '>=' ) &&
+	! class_exists( 'PHPUnit_Framework_TestCase' ) ) {
 	class_alias( 'PHPUnit\Framework\TestCase',                   'PHPUnit_Framework_TestCase' );
 	class_alias( 'PHPUnit\Framework\Exception',                  'PHPUnit_Framework_Exception' );
 	class_alias( 'PHPUnit\Framework\ExpectationFailedException', 'PHPUnit_Framework_ExpectationFailedException' );