Try checking for an existing compat file.

Author: octalmage

tests/phpunit/bootstrap.php

@@ -21,11 +21,6 @@
 	$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';
-}
-
 if ( file_exists( $test_root . '/includes/functions.php' ) ) {
 	require_once $test_root . '/includes/functions.php';
 	function _manually_load_plugin() {
@@ -35,4 +30,12 @@ function _manually_load_plugin() {
 	tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );
 }
 
+// Core started including their own phpunit6-compat, so we should use that if we can.
+if ( ! file_exists( $test_root . '/includes/phpunit6-compat.php' ) ) {
+	// 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';
+	}
+}
+
 require $test_root . '/includes/bootstrap.php';

tests/phpunit/phpunit6-compat.php

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