[RIP-10] Add test cases for QueryMsgByUniqueKeySubCommand

[codefans]

What is the purpose of the change

   Add test cases for QueryMsgByUniqueKeySubCommand

Brief changelog

   Add QueryMsgByUniqueKeySubCommandTest.java

Verifying this change

   run QueryMsgByUniqueKeySubCommandTest.java

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[xiangwangcheng]

The test case shouldn't depend on specific values like "127.0.0.1:9876"(namesrv addr) and "myTopicTest"(topic name). That makes the test case fail on others' PC.

[coveralls]

Coverage decreased (-0.2%) to 48.808% when pulling fd19e35 on codefans:develop into 4f8d96c on apache:develop.

[hdchen]

LGTM

[zongtanghu]

This pr add the test case for QueryMsgByUniqueKeySubCommand,but CI code coverage is down.Can you find the reason by looking at the CI code coverate report?