[type: test] Add unit test for NacosDiscoveryService

[eurecalulu]

related to #5278

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (928c802) 62.06% compared to head (2dae720) 61.96%.

Files	Patch %	Lines
...he/shenyu/admin/model/enums/DiscoveryTypeEnum.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5310      +/-   ##
============================================
- Coverage     62.06%   61.96%   -0.11%     
+ Complexity     8682     8670      -12     
============================================
  Files          1252     1253       +1     
  Lines         38064    38191     +127     
  Branches       3645     3656      +11     
============================================
+ Hits          23625    23664      +39     
- Misses        12348    12423      +75     
- Partials       2091     2104      +13     

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