Test PR 3769 (Improves the way LMS serves /favicon.ico)

Because this might need a little discussion, I'm making a separate pull request for these tests for #3769 

In particular:

1. is this the best way to have dummy theme templates? (required to test the case where `USE_CUSTOM_THEME` is `True`)
2. is there any way to avoid a 404 on the redirects? It doesn't prevent the tests from doing their job, but it would be nice if the code on redirect was 200 (the issue is that, in testing, `/static/` doesn't serve up static media.