[dubbo-1689]: Enhance the test coverage part-10 : dubbo-plugin module

[beiwei30]

What is the purpose of the change

#1689: Enhance the test coverage part-10 : dubbo-plugin module

Brief changelog

dubbo-plugin/dubbo-qos/src/test/java/org/apache/dubbo/qos/command/CommandContextFactoryTest.java
dubbo-plugin/dubbo-qos/src/test/java/org/apache/dubbo/qos/command/CommandContextTest.java
dubbo-plugin/dubbo-qos/src/test/java/org/apache/dubbo/qos/command/DefaultCommandExecutorTest.java
dubbo-plugin/dubbo-qos/src/test/java/org/apache/dubbo/qos/command/GreetingCommand.java
dubbo-plugin/dubbo-qos/src/test/resources/META-INF/services/org.apache.dubbo.qos.command.BaseCommand

Verifying this change

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue filed 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.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. 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.
• 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 integration-test in test module.
• Run mvn clean install -DskipTests & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-io]

Codecov Report

Merging #1949 into master will increase coverage by 0.65%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1949      +/-   ##
============================================
+ Coverage      50.4%   51.05%   +0.65%     
- Complexity     4741     4869     +128     
============================================
  Files           518      532      +14     
  Lines         25424    25833     +409     
  Branches       4560     4615      +55     
============================================
+ Hits          12814    13190     +376     
- Misses        10655    10661       +6     
- Partials       1955     1982      +27

Impacted Files	Coverage Δ	Complexity Δ
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	87.85% <0%> (-1.87%)	15% <0%> (ø)
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	56.81% <0%> (ø)	8% <0%> (?)
...dubbo/remoting/p2p/support/MulticastNetworker.java	100% <0%> (ø)	2% <0%> (?)
...ubbo/remoting/p2p/exchange/ExchangeNetworkers.java	0% <0%> (ø)	0% <0%> (?)
...g/apache/dubbo/remoting/p2p/support/FileGroup.java	37.03% <0%> (ø)	5% <0%> (?)
...g/p2p/exchange/support/MulticastExchangeGroup.java	76.59% <0%> (ø)	7% <0%> (?)
...oting/p2p/exchange/support/ExchangeServerPeer.java	26.53% <0%> (ø)	6% <0%> (?)
...java/org/apache/dubbo/remoting/p2p/Networkers.java	83.33% <0%> (ø)	4% <0%> (?)
...p/exchange/support/MulticastExchangeNetworker.java	100% <0%> (ø)	2% <0%> (?)
...moting/p2p/exchange/support/FileExchangeGroup.java	0% <0%> (ø)	0% <0%> (?)
... and 21 more

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