Move `channel_name` to Channel.broadcasting_for

That would allow us to test broadcasting made with channel, e.g.:

```ruby
class ChatRelayJob < ApplicationJob
  def perform_later(room, msg)
    ChatChannel.broadcast_to room, message: msg
  end
end
```

To test this functionality we need to know the underlying stream name
(to use `assert_broadcasts`), which relies on `channel_name`.

We had to use the following code:

```ruby
assert_broadcasts(ChatChannel.broadcasting_for([ChatChannel.channel_name, room]), 1) do
  ChatRelayJob.perform_now
end
```

The problem with this approach is that we use _internal_ API (we shouldn't care about `channel_name` prefix
in our code).

With this commit we could re-write the test as following:

```ruby
 assert_broadcasts(ChatChannel.broadcasting_for(room), 1) do
   ChatRelayJob.perform_now
 end
```

1 files changed, 27 insertions, 0 deletions

diff --git a/guides/source/testing.md b/guides/source/testing.md
index 1a2f480407..fb78fa6d17 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -1837,6 +1837,33 @@ class ProductTest < ActionCable::TestCase
 end
 ```
 
+If you want to test the broadcasting made with `Channel.broadcast_to`, you shoud use
+`Channel.broadcasting_for` to generate an underlying stream name:
+
+```ruby
+# app/jobs/chat_relay_job.rb
+class ChatRelayJob < ApplicationJob
+  def perform_later(room, message)
+    ChatChannel.broadcast_to room, text: message
+  end
+end
+
+# test/jobs/chat_relay_job_test.rb
+require 'test_helper'
+
+class ChatRelayJobTest < ActiveJob::TestCase
+  include ActionCable::TestHelper
+
+  test "broadcast message to room" do
+    room = rooms[:all]
+
+    assert_broadcast_on(ChatChannel.broadcasting_for(room), text: "Hi!") do
+      ChatRelayJob.perform_now(room, "Hi!")
+    end
+  end
+end
+```
+
 Additional Testing Resources
 ----------------------------