Add isSubscribed method to AbstractTripleMutinyPublisher for unit test

[zrlw]

What is the purpose of the change?

try to fix #15660

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[zrlw]

duplicated with #15662

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.00%. Comparing base (5b9adb0) to head (f4a3314).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15661      +/-   ##
============================================
- Coverage     61.06%   61.00%   -0.06%     
+ Complexity    11704    11698       -6     
============================================
  Files          1923     1923              
  Lines         87069    87070       +1     
  Branches      13112    13112              
============================================
- Hits          53165    53115      -50     
- Misses        28465    28501      +36     
- Partials       5439     5454      +15     

Flag	Coverage Δ
integration-tests-java21	?
integration-tests-java8	33.01% <0.00%> (+0.05%)	⬆️
samples-tests-java21	32.67% <0.00%> (+0.02%)	⬆️
samples-tests-java8	30.36% <0.00%> (+<0.01%)	⬆️
unit-tests-java11	58.99% <ø> (-0.03%)	⬇️
unit-tests-java17	58.74% <100.00%> (-0.03%)	⬇️
unit-tests-java21	58.74% <100.00%> (-0.03%)	⬇️
unit-tests-java8	59.01% <ø> (+0.01%)	⬆️

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