Fix non-deterministic tests for issue#3149

[Mukta13]

What this PR does / why we need it

This PR addresses non-deterministic behavior in two specific tests, ensuring consistent and predictable results across builds.

Summary of Changes

• Modified Flaky Tests:

  • Issue2959.java:

    • Updated test() to enforce a consistent ordering of fields in the expected output, matching it with the actual output format.

  • DubboTest4.java:

    • Adjusted test6() to handle expected exceptions properly, accounting for specific exceptions (e.g., java.io.IOException) rather than returning null.

Testing Instructions

The modifications can be validated using the NonDex plugin to simulate non-deterministic behavior:

mvn -pl core edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=com.alibaba.fastjson2.issues_2900.Issue2959
mvn -pl core edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=com.alibaba.fastjson2.dubbo.DubboTest4#test6

Checklist

• Ensured all tests are passing and test coverage is added where necessary.
• Verified that commit messages adhere to the Conventional Commits specification.
• Considered the documentation impact, and if needed, created or updated documentation accordingly.