cleans up logo integration tests

the 3rd and final step of the refactoring done in these two pull requests: https://github.com/discourse/discourse/pull/1587, https://github.com/discourse/discourse/pull/1595

I have removed all the integration tests for logo (there is now an identical set of unit tests for the logo component so they are no longer needed) and replaced them with one new integration test, that checks that the logo component is really rendered in the header and correctly wired to property passed from header controller.