What is the purpose of the change

fix #6749

Brief changelog

Brief changelog:
1.dubbo-common/src/test/java/org/apache/dubbo/common/config/configcenter/ConfigChangedEventTest.java
2.dubbo-config-spring/src/test/java/org/apache/dubbo/config/spring/registry/MockRegistryFactory.java

Verifying this change

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• Make sure there is a #6749 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.
• 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 sample in dubbo samples project.
• Run mvn clean install -DskipTests=false & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
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