test(#3269636): add failing test for private:// redirect path bug

Stuart Clarkrequested to merge
issue/filefield_paths-3269636:feature/3269636-private_redirect_path into 8.x-1.x

Adds a Kernel test that reproduces the bug reported in this issue: a redirect to a private:// destination resolves to the raw filesystem path instead of the /system/files/... route.

This MR is test-only and the new test is expected to fail. A follow-up MR will fix Redirect::getPath() and this test should then pass.

Merge request reports