Add unit tests to package com.alibaba.nacos.naming.core in nacos 2.0

[realJackSun]

sub-issue of #4982 The coverage rate of com.alibaba.nacos.naming.core is just 34%, could you please provide more unit tests?

Anything else we need to know?

To-do list:

• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.cleaner #5520
• [x] Add unit tests for package com.alibaba.nacos.naming.core.v2.client
• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.event
• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.index #5521
• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.metadata #5522
• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.service
• [ ] Add unit tests for package com.alibaba.nacos.naming.core.v2.upgrade
• [ ] other classes under com.alibaba.nacos.naming.core.*

[realJackSun]

@I will solve it@