Let `open_browser` accept 2nd arg `open_fun`

[peaceful-james]

Same as https://hexdocs.pm/phoenix_live_view/Phoenix.LiveViewTest.html#open_browser/2

[germsvel]

@peaceful-james I'd be happy to review a PR if you want to introduce one. We just need to make sure to cover the LiveView and Static cases.

Do you mind me asking, what's your use case? I haven't ever needed to pass a function to open_browser except when trying to test the function itself.