[Subtask]: Improve the unit tests for module Flink

[lklhdu]

Search before asking

• [X] I have searched in the issues and found no similar issues.

What would you like to be improved?

Unit testing is an important part of ensuring the quality of open-source projects and we need to improve unit test cases in Flink module.

Parent issue

#937

Are you willing to submit PR?

• [ ] Yes I am willing to submit a PR!

Subtasks

• [ ] #1364
• [x] #1440
• [x] #1596
• [ ] #1597
• [x] #696

Code of Conduct

• [X] I agree to follow this project's Code of Conduct