Add unit tests for `dynamic-datasource-spring-boot3-starter`

[linghengqian]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style
• [ ] Refactor
• [ ] Doc
• [ ] Other, please describe:

The description of the PR:

• For https://github.com/baomidou/dynamic-datasource/issues/475 .
• Add unit tests for dynamic-datasource-spring-boot3-starter.
• For https://github.com/baomidou/dynamic-datasource/issues/465, we need some reasonable unit tests to complete nativeTest validation in the future to avoid breaking binary compatibility during the refactoring process.

Other information:

• Null.

[linghengqian]

• According to https://github.com/baomidou/dynamic-datasource/actions/runs/5927925719/job/16072452561?pr=552, it is no longer possible to build JDK7-usable artifacts using the version of Maven Wrapper used by the project.

• @huayanYu Should I update the README banner? This resulted in the fact that it is no longer testable.

[huayanYu]

@linghengqian 可以的

[linghengqian]

@linghengqian 可以的

• @huayanYu Done. Please help review.