Add unit test for TraceTelnetHandler (#7843)

[icankeep]

What is the purpose of the change

issue: #7843

Brief changelog

Add unit test for TraceTelnetHandler

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
• GitHub Actions works fine on your own branch.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-commenter]

Codecov Report

Merging #7861 (53694fe) into master (2dace54) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #7861      +/-   ##
============================================
+ Coverage     59.60%   59.61%   +0.01%     
- Complexity      556      557       +1     
============================================
  Files          1088     1088              
  Lines         43854    43854              
  Branches       6400     6400              
============================================
+ Hits          26140    26145       +5     
+ Misses        14814    14790      -24     
- Partials       2900     2919      +19     

Impacted Files	Coverage Δ
...rg/apache/dubbo/remoting/utils/PayloadDropper.java	76.92% <0.00%> (-15.39%)	⬇️
.../apache/dubbo/rpc/protocol/AsyncToSyncInvoker.java	62.06% <0.00%> (-10.35%)	⬇️
.../dubbo/remoting/transport/netty4/NettyChannel.java	61.38% <0.00%> (-2.98%)	⬇️
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	86.66% <0.00%> (-1.91%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	78.14% <0.00%> (-1.49%)	⬇️
...he/dubbo/remoting/transport/netty/NettyServer.java	73.68% <0.00%> (+3.50%)	⬆️
...e/dubbo/rpc/protocol/dubbo/filter/TraceFilter.java	22.64% <0.00%> (+9.43%)	⬆️
...rg/apache/dubbo/qos/legacy/TraceTelnetHandler.java	32.43% <0.00%> (+32.43%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2dace54...53694fe. Read the comment docs.