add test cases for ShenyuClientApiDocExecutorSubscriber class.

[omernaci]

Related to #4544

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

Merging #4632 (89cce8f) into master (c183bf9) will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4632      +/-   ##
============================================
+ Coverage     67.68%   67.72%   +0.03%     
- Complexity     7957     7962       +5     
============================================
  Files          1094     1094              
  Lines         31207    31207              
  Branches       2812     2812              
============================================
+ Hits          21124    21135      +11     
+ Misses         8426     8414      -12     
- Partials       1657     1658       +1     

see 4 files with indirect coverage changes

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[li-keguo]

Thank you for your contribution