Ut telnet handler adapter test

[dgzpg]

What is the purpose of the change

add test for TelnetHandlerAdapter
see more detail from #8257

Brief changelog

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.
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[codecov-commenter]

Codecov Report

Merging #8258 (33997c0) into master (11e15f2) will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #8258      +/-   ##
============================================
+ Coverage     61.24%   61.28%   +0.04%     
  Complexity      489      489              
============================================
  Files          1090     1090              
  Lines         43963    43963              
  Branches       6420     6420              
============================================
+ Hits          26924    26944      +20     
+ Misses        14063    14049      -14     
+ Partials       2976     2970       -6     

Impacted Files	Coverage Δ
...ubbo/registry/support/AbstractRegistryFactory.java	83.09% <0.00%> (-2.82%)	⬇️
...e/dubbo/remoting/transport/netty4/NettyClient.java	69.56% <0.00%> (-1.45%)	⬇️
.../src/main/java/org/apache/dubbo/rpc/RpcStatus.java	72.61% <0.00%> (-1.20%)	⬇️
...ting/zookeeper/curator/CuratorZookeeperClient.java	68.96% <0.00%> (-1.15%)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	79.72% <0.00%> (-0.35%)	⬇️
.../dubbo/remoting/transport/netty4/NettyChannel.java	62.37% <0.00%> (+0.99%)	⬆️
...pache/dubbo/registry/support/AbstractRegistry.java	79.62% <0.00%> (+1.11%)	⬆️
...ng/transport/dispatcher/all/AllChannelHandler.java	96.55% <0.00%> (+6.89%)	⬆️
...o/remoting/transport/ChannelHandlerDispatcher.java	70.00% <0.00%> (+10.00%)	⬆️
.../remoting/telnet/support/TelnetHandlerAdapter.java	88.63% <0.00%> (+36.36%)	⬆️

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[AlbumenJ]

Thanks for your contributions. Please also pick these changes to 3.0 branch.

[dgzpg]

Thanks for your contributions. Please also pick these changes to 3.0 branch.

OK! thx!