add UT for namingserver module

- [ ] I have searched the [issues](https://github.com/seata/seata/issues) of this repository and believe that this is not a duplicate.

### Ⅰ. Issue Description

https://app.codecov.io/gh/apache/incubator-seata/tree/2.x/namingserver%2Fsrc%2Fmain%2Fjava%2Forg%2Fapache%2Fseata%2Fnamingserver


### Ⅱ. Describe what happened

If there is an exception, please attach the exception trace:

```
Just paste your stack trace here!
```

### Ⅲ. Describe what you expected to happen

### Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. xxx
2. xxx
3. xxx

Minimal yet complete reproducer code (or URL to code):

### Ⅴ. Anything else we need to know?

### Ⅵ. Environment:

- JDK version(e.g. `java -version`):
- Seata client/server version:
- Database version:
- OS(e.g. `uname -a`):
- Others: