add favicon to fix flaky tests (#2082)

We've recently been seeing some flaky system test failures such as
https://github.com/ViewComponent/view_component/actions/runs/10582240672/job/29321458276?pr=2081
with the error `No route matches [GET] "/favicon.ico"`. It appears
as though the browser is trying to load a favicon and when it fails
to load before the test exits, causes the test to fail.