fix(php) SupportPHP 5.6.

Author: Hywan

classes/test.php

@@ -268,7 +268,10 @@ public function getTestedClassName()
         // scores will not work.
         if (true === $this->breakRelationBetweenTestSuiteAndSUT &&
             $this instanceof PHPUnit\Framework\TestCase) {
-            return $this->defaultTestedClassName ?? 'StdClass';
+            return
+                null !== $this->defaultTestedClassName
+                    ? $this->defaultTestedClassName
+                    : 'StdClass';
         }
 
         $testedClassName = parent::getTestedClassName();