[3.0] Add unit test for TraceTelnetHandler

[icankeep]

What is the purpose of the change

issue: #7843
pick from master d6d8d25d

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 #7909 (8015e44) into 3.0 (ca2e351) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.0    #7909      +/-   ##
============================================
+ Coverage     58.89%   58.92%   +0.02%     
- Complexity      324      326       +2     
============================================
  Files          1081     1081              
  Lines         43840    43840              
  Branches       6536     6536              
============================================
+ Hits          25820    25832      +12     
+ Misses        15154    15132      -22     
- Partials       2866     2876      +10     

Impacted Files	Coverage Δ
.../remoting/transport/netty4/NettyServerHandler.java	61.36% <0.00%> (-9.10%)	⬇️
...rg/apache/dubbo/registry/dns/util/DNSResolver.java	68.88% <0.00%> (-4.45%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	76.92% <0.00%> (-1.47%)	⬇️
...he/dubbo/registry/multicast/MulticastRegistry.java	67.43% <0.00%> (-0.92%)	⬇️
...he/dubbo/registry/zookeeper/ZookeeperRegistry.java	57.86% <0.00%> (-0.63%)	⬇️
...apache/dubbo/common/extension/ExtensionLoader.java	76.24% <0.00%> (-0.22%)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	63.79% <0.00%> (+1.03%)	⬆️
...e/dubbo/remoting/transport/netty/NettyChannel.java	60.22% <0.00%> (+7.95%)	⬆️
.../apache/dubbo/remoting/transport/AbstractPeer.java	67.39% <0.00%> (+8.69%)	⬆️
...rg/apache/dubbo/qos/legacy/TraceTelnetHandler.java	32.43% <0.00%> (+32.43%)	⬆️

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[xiaoheng1]

LGTM