Clean up after #6288 (comment)

Author: sebastianbergmann

src/Framework/TestCase.php

@@ -367,7 +367,7 @@ final public function run(): void
             return;
         }
 
-        IsolatedTestRunnerRegistry::run(
+        (new SeparateProcessTestRunner)->run(
             $this,
             $this->runClassInSeparateProcess && !$this->runTestInSeparateProcess,
             $this->preserveGlobalState,

src/Framework/TestRunner/IsolatedTestRunner.php

@@ -34,7 +34,7 @@
  *
  * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
-final class SeparateProcessTestRunner implements IsolatedTestRunner
+final class SeparateProcessTestRunner
 {
     private static ?string $sourceMapFile = null;