fix TestCase::onNotSuccessfulTest() for PHP 8.0 and lower

src/Phpunit/TestCase.php

@@ -13,11 +13,30 @@
 use PHPUnit\Util\Test as TestUtil;
 use SebastianBergmann\CodeCoverage\CodeCoverage;
 
+if (\PHP_VERSION_ID < 81_000) {
+    trait Phpunit9xTestCaseTrait
+    {
+        protected function onNotSuccessfulTest(\Throwable $e): void
+        {
+            $this->_onNotSuccessfulTest($e);
+        }
+    }
+} else {
+    trait Phpunit9xTestCaseTrait
+    {
+        protected function onNotSuccessfulTest(\Throwable $e): never
+        {
+            $this->_onNotSuccessfulTest($e);
+        }
+    }
+}
+
 /**
  * Generic TestCase for PHPUnit tests for ATK4 repos.
  */
 abstract class TestCase extends BaseTestCase
 {
+    use Phpunit9xTestCaseTrait;
     use WarnDynamicPropertyTrait;
 
     protected function setUp(): void
@@ -171,7 +190,7 @@ private function releaseObjectsFromExceptionTrace(\Throwable $e): void
         }
     }
 
-    protected function onNotSuccessfulTest(\Throwable $e): void
+    protected function _onNotSuccessfulTest(\Throwable $e): void
     {
         // release objects from uncaught exception as it is never released
         $this->releaseObjectsFromExceptionTrace($e);