added tests for siren-entity-loading.html (polymer 3 modulizer only converts files that are imported, and this file was only imported by all-imports, which is deleted in our automation prior to the modulizer running, so it seemed like a good time to add some more tests)
There are currently 3 remaining IE11 failures that seem to be due to some sort of polyfill issue. My plan is to ignore them for now and revisit if they are still an issue post-polymer 3 conversion.