Fix child test (should have never worked)

But it did because the child process output was "childTjs=" but this is incorrect and cannot be unserialized.

tests/ChildRuntimeTest.php

@@ -28,7 +28,10 @@ public function it_can_run()
         $process->start();
 
         $process->wait();
+        $output = unserialize(base64_decode($process->getOutput()));
 
-        $this->assertStringContainsString('child', $process->getOutput());
+        $this->assertIsArray($output);
+        $this->assertArrayHasKey('output', $output);
+        $this->assertStringContainsString('child', $output['output']);
     }
 }