Kill BackgroundCluster if it does not shut down

src/gwf/backends/local.py

@@ -459,7 +459,8 @@ def start(cls, working_dir, max_cores, host, port, **kwargs):
     def shutdown(self):
         with Client.connect("localhost", 12345) as c:
             c.shutdown()
-        self.process.join()
+        self.process.join(timeout=1)
+        self.process.kill()
 
     def __enter__(self):
         return self