add mock test for seata-discovery-nacos module

[slievrly]

• [ ] I have searched the 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/discovery%2Fseata-discovery-nacos%2Fsrc%2Fmain%2Fjava%2Forg%2Fapache%2Fseata%2Fdiscovery%2Fregistry%2Fnacos

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:

[mehedikhan72]

Hi, @slievrly . I've added some mock tests in this PR. Can you take a look maybe? Once mock tests for this module is accepted, I'll do the other discovery modules.

[slievrly]

@mehedikhan72 thanks for your contribution.