[thread-cert] fix bug that docker network is not removed (#10590)

In some cases that `_do_packet_verification` is False (e.g. verify() is
not defined), the docker network is not correctly removed.

docker network interface should always be removed at the end of the test
if it was created at the begin of the test

tests/scripts/thread-cert/thread_cert.py

@@ -296,10 +296,12 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
 
         self.simulator.stop()
 
+        if self._has_backbone_traffic():
+            self._remove_backbone_network()
+
         if self._do_packet_verification:
 
             if self._has_backbone_traffic():
-                self._remove_backbone_network()
                 pcap_filename = self._merge_thread_backbone_pcaps()
             else:
                 pcap_filename = self._get_thread_pcap_filename()