optimize: optimize NacosConfiguration singleton reload

[slievrly]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

add Nacos mock test

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Attention: Patch coverage is 59.09091% with 9 lines in your changes missing coverage. Please review.

Project coverage is 50.84%. Comparing base (f7c3866) to head (abc5120).
Report is 2 commits behind head on 2.x.

Files	Patch %	Lines
.../apache/seata/config/nacos/NacosConfiguration.java	59.09%	2 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #6763      +/-   ##
============================================
+ Coverage     50.57%   50.84%   +0.26%     
- Complexity     5936     5970      +34     
============================================
  Files          1061     1063       +2     
  Lines         37123    37153      +30     
  Branches       4404     4408       +4     
============================================
+ Hits          18776    18889     +113     
+ Misses        16445    16339     -106     
- Partials       1902     1925      +23     

Files	Coverage Δ
.../apache/seata/config/nacos/NacosConfiguration.java	41.88% <59.09%> (+29.73%)	⬆️

... and 7 files with indirect coverage changes

[funky-eyes]

LGTM