[3.3] optimized testExporterThreadpoolName by starting dispatcher thread directly

[zrlw]

What is the purpose of the change?

remove redundant thread creating and time waiting.
see details at https://github.com/apache/dubbo/actions/runs/13841589593/job/38730837928

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?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.77%. Comparing base (d3b1e1f) to head (c4fc650).
⚠️ Report is 174 commits behind head on 3.3.

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15241      +/-   ##
============================================
- Coverage     60.79%   60.77%   -0.02%     
+ Complexity    10892    10887       -5     
============================================
  Files          1885     1885              
  Lines         86082    86087       +5     
  Branches      12895    12896       +1     
============================================
- Hits          52331    52323       -8     
- Misses        28303    28311       +8     
- Partials       5448     5453       +5     

Flag	Coverage Δ
integration-tests	33.13% <ø> (-0.02%)	⬇️
samples-tests	29.20% <ø> (+<0.01%)	⬆️
unit-tests	58.91% <ø> (-0.01%)	⬇️

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