Autoload: improve/stabilize version check for TestCase

The version check in the `Autoload::loadTestCase()` method could get confused if the package using this library, or a dependency of the package using this library would alias the `PHPUnit_Runner_Version`/`PHPUnit\Runner\Version` class.

As both classes contain the method we need to retrieve the version number, let's always base the loading on the available PHPUnit version number instead of the availability of either class.

phpunitpolyfills-autoload.php

@@ -3,6 +3,7 @@
 namespace Yoast\PHPUnitPolyfills;
 
 use PHPUnit\Runner\Version as PHPUnit_Version;
+use PHPUnit_Runner_Version;
 
 if ( \class_exists( 'Yoast\PHPUnitPolyfills\Autoload', false ) === false ) {
 
@@ -426,9 +427,7 @@ public static function loadAssertObjectEquals() {
 		 * @return void
 		 */
 		public static function loadTestCase() {
-			if ( \class_exists( '\PHPUnit_Runner_Version' ) === true
-				|| \version_compare( PHPUnit_Version::id(), '8.0.0', '<' )
-			) {
+			if ( \version_compare( self::getPHPUnitVersion(), '8.0.0', '<' ) ) {
 				// PHPUnit < 8.0.0.
 				require_once __DIR__ . '/src/TestCases/TestCasePHPUnitLte7.php';
 				return;
@@ -444,7 +443,7 @@ public static function loadTestCase() {
 		 * @return void
 		 */
 		public static function loadTestListenerDefaultImplementation() {
-			if ( \class_exists( '\PHPUnit_Runner_Version' ) === true ) {
+			if ( \version_compare( self::getPHPUnitVersion(), '6.0.0', '<' ) ) {
 				/*
 				 * Alias one particular PHPUnit 4/5 class to its PHPUnit >= 6 name.
 				 *
@@ -474,6 +473,26 @@ public static function loadTestListenerDefaultImplementation() {
 			// PHPUnit >= 7.0.0.
 			require_once __DIR__ . '/src/TestListeners/TestListenerDefaultImplementationPHPUnitGte7.php';
 		}
+
+		/**
+		 * Retrieve the PHPUnit version id.
+		 *
+		 * As both the pre-PHPUnit 6 class, as well as the PHPUnit 6+ class contain the `id()` function,
+		 * this should work independently of whether or not another library may have aliased the class.
+		 *
+		 * @return string Version number as a string.
+		 */
+		public static function getPHPUnitVersion() {
+			if ( \class_exists( '\PHPUnit\Runner\Version' ) ) {
+				return PHPUnit_Version::id();
+			}
+
+			if ( \class_exists( '\PHPUnit_Runner_Version' ) ) {
+				return PHPUnit_Runner_Version::id();
+			}
+
+			return '0';
+		}
 	}
 
 	\spl_autoload_register( __NAMESPACE__ . '\Autoload::load' );