I'm not keen on the `__call__` either, but I'm ok with it for the following reasons:
1. It's in an app object, not a test, so it's less likely to get in the way of/cause confusion for someone who is a new contributor to gaia-ui-tests.
2. This one is in the browser app object, which @zacc has said is going away, so it seems fine as a stop gap for now.