[ip6] set Mesh Local IID for Reference Devices (#6599)

tests/scripts/thread-cert/node.py

@@ -2622,6 +2622,11 @@ class NodeImpl:
 
         return result
 
+    def set_mliid(self, mliid: str):
+        cmd = f'mliid {mliid}'
+        self.send_command(cmd)
+        self._expect_command_output(cmd)
+
 
 class Node(NodeImpl, OtCli):
     pass