Replace existing tests that target unexported functions with tests for exported

From https://github.com/mgechev/revive/pull/1353#issuecomment-2887129427:

I'm not sure we should test unexported functions. They are internals and a matter to be freely changed with the refactoring. Testing them makes code less flexible for refactoring and tests more fragile. In my current project, for example, we have this strongly discouraged (with some legacy exceptions).

To improve test structure and enforce better package boundaries, we should refactor our current test suite:

1. Replace existing tests that target unexported functions with equivalent tests targeting exported functions. Ensure the functionality is still covered by indirectly testing through public APIs.
2. Adopt _test package convention.
3. Enable testpackage linter in the golangci-lint config.