integration tests for the logo part of header

I'm aware that these tests are too detailed (and therefore too slow) for integration tests, but I intend it as a temporary step of a refactoring.

I plan to extract the logo generation logic to a separate component. It'll be a 3 step process:
- covering logo with the integration tests (this pull request)
- extracting logo to a separate component (with its own unit tests) - integration tests from the previous step will provide a safety net for the extraction that will prove that the logic is extracted 1:1
- removing (now redundant) integration tests, leaving only matching set of newly extracted component's unit tests