Refactor org.apache.dubbo.reactive.test

[gzhao9]

Purpose of the Change

This PR is aimed at resolving the issue of duplicated creation of mock. This occurs in five test cases in ManyToManyMethodHandlerTest.java, ManyToOneMethodHandlerTest.java and OneToManyMethodHandlerTest.java, as highlighted in issue #13154.

Brief changelog

We created a reusable class creatObserverAdapter to create the mock, avoiding duplicate creation of the mock in 5 test cases.
Since in ManyToOneMethodHandlerTest.java and OneToManyMethodHandlerTest.java, the mock is both created duplicated in two test cases, we set this mock object as a class attribute and create it in @BeforeEach.
After refactoring, we removed duplicate lines of code for the following five test cases and improved readability and maintainability:

• ManyToManyMethodHandlerTest.java
  • testInvoke() Lines 43-52
• ManyToOneMethodHandlerTest.java
  • testInvoke() Lines 43-52
  • testError() Lines 68-77
• OneToManyMethodHandlerTest.java
  • testInvoke() Lines 43-52
  • testError() Lines 65-74

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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