This is part several updates for #22524.
Testing note: I removed the test test_compose_all_everyone_confirm_clicked
since it heavily relies on the old way of rendering banners and is too
UI-focused (instead of logic focused) for me to feel like it's worth
testing that the banner is removed when clicking the button.
This may have originally made sense as a class that managed the
browser state, but it has since turned into a dumping ground for
mostly pure functions that don’t make sense to instantiate.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
The only downside of this is that it makes it harder to control the
order of these tests; which isn't that important. And the structure
of naming each with its test order fundamentally requires renaming
files when adding/deleting tests, so if we want to control the default
test order, we'd be better off doing that by just hardcoding a list in
the test runner code.