Make ActionController::Base.asset_host point to spec/javascripts/fixtures/ when generating JavaScript test fixtures

The fixtures we generate for our JavaScript tests with bin/rake karma:fixtures contain over 1000 references to http://test.host/assets/...:

grep -orE 'http://test.host/assets/[^"]+\.(png|svg)' spec/javascripts/fixtures/

All of these lead to 404s when running the JavaScript tests. We should override ActionController::Base.asset_host when generating the fixtures so that assets instead point to one or more files in spec/javascripts/fixtures/.