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

[shalk]

Issue Description

This is a sub-issue of [ISSUE #5011]
The package com.alibaba.nacos.client.naming coverage rate is just 5%, Could you add more unit tests to nacos-client module?

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To-do list:

• Add unit tests for package com.alibaba.nacos.client.naming.backups
• Add unit tests for package com.alibaba.nacos.client.naming.beat
• Add unit tests for package com.alibaba.nacos.client.naming.cache
• Add unit tests for package com.alibaba.nacos.client.naming.core
• Add unit tests for package com.alibaba.nacos.client.naming.event
• Add unit tests for package com.alibaba.nacos.client.naming.remote
• Add unit tests for package com.alibaba.nacos.client.naming.utils
• Add unit tests for package com.alibaba.nacos.client.naming.* other class