User correct PHPUnit 6 compat file path

It has changed on more recent versions of WordPress.

See #27

src/WPPPB/Loader.php

@@ -227,14 +227,29 @@ public function locate_wp_tests_config() {
 	public function phpunit_compat() {
 
 		if ( class_exists( 'PHPUnit\Runner\Version' ) ) {
-
-			/**
-			 * Compatibility with PHPUnit 6+.
-			 *
-			 * @since 0.3.2
-			 */
-			require_once $this->get_wp_tests_dir() . '/includes/phpunit6-compat.php';
-
+
+			$tests_dir = $this->get_wp_tests_dir();
+
+			// Back-compat for older WP versions.
+			if ( file_exists( $tests_dir . '/includes/phpunit6-compat.php' ) ) {
+
+				/**
+				 * Compatibility with PHPUnit 6+.
+				 *
+				 * @since 0.3.2
+				 */
+				require_once $tests_dir . '/includes/phpunit6-compat.php';
+
+			} else {
+
+				/**
+				 * Compatibility with PHPUnit 6+.
+				 *
+				 * @since 0.3.6
+				 */
+				require_once $tests_dir . '/includes/phpunit6/compat.php';
+			}
+
 			class_alias(
 				'PHPUnit\Framework\Constraint\Constraint'
 				, 'PHPUnit_Framework_Constraint'