[thread-cert] misc enhancements to improve reliability (#7298)

This commit introduces a number of enhancements to `thread-cert`
Border Router tests to improve reliability:
- Checks `ot-rcp` process status for OTBR tests to help catch errors
  earlier
- Make sure `ot-rcp` processes are terminated after one test to avoid
  corrupting subsequent tests
- Upload core dump as artifacts if otbr-agent crashed

tests/scripts/thread-cert/thread_cert.py

@@ -269,8 +269,12 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             self._stop_backbone_sniffer()
 
         for node in list(self.nodes.values()):
-            node.stop()
-            node.destroy()
+            try:
+                node.stop()
+            except:
+                traceback.print_exc()
+            finally:
+                node.destroy()
 
         self.simulator.stop()