Supplement and improvement for test cases

## Issue Description

Type: *proposal (call for all)*

### Describe what happened (or what feature you want)

In order to guarantee the code quality and avoid potential bugs, we now call for all developers to improve tests of Sentinel. Any kinds of enhancement is welcomed, including (but not limited to):

- add more test cases (including unit test and integration test)
- improve existing tests

We use [codecov](https://codecov.io/gh/alibaba/Sentinel) to measure the test coverage of the project. Everyone can refer to [here](https://codecov.io/gh/alibaba/Sentinel) to pick modules with low-coverage and add test cases for it.

Let's hack to avoid potential bugs and improve our code quality!

Start with the Sentinel modules shown as low coverage on Codecov and inspect their existing unit and integration tests. Choose a specific coverage gap before making changes. Done means adding or improving tests that cover the selected behavior and confirming the project tests and coverage results pass.

Written by the indexing model from the issue text.