This PR is similar to https://github.com/rails/rails/pull/47025, it makes Action Cable's `assert_broadcasts` return the messages that were broadast. This way you can do more analysis on them:

```ruby
    messages = assert_broadcasts("test", 2) do
      ActionCable.server.broadcast "test", { message: "one" }
      ActionCable.server.broadcast "test", { message: "two" }
    end
    assert_equal 2, messages.length
    assert_equal({ "message" => "one" }, messages.first)
    assert_equal({ "message" => "two" }, messages.last)
```

This is helpful if you expect lots of messages to be broadcast or if you want to only match on some element of the data; `assert_broadcast_on` doesn't work well in either of those scenarios.