autoload: always load Yii.php to fix 'Class Yii not found' error

Error: Class 'Yii' not found

.../yii2-client-cert-auth/vendor/yiisoft/yii2/base/Application.php:196
.../yii2-client-cert-auth/tests/TestCase.php:19
.../yii2-client-cert-auth/tests/SubjectTest.php:25
/usr/share/php/PHPUnit/TextUI/Command.php:155
/usr/share/php/PHPUnit/TextUI/Command.php:106

composer.json

@@ -18,6 +18,9 @@
     "autoload": {
         "psr-4": {
             "fphammerle\\yii2\\auth\\clientcert\\": ""
-        }
+        },
+        "files": [
+            "vendor/yiisoft/yii2/Yii.php"
+        ]
     }
 }