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, 12 insertions, 3 deletions

diff --git a/actioncable/test/channel/broadcasting_test.rb b/actioncable/test/channel/broadcasting_test.rb
index 2cbfabc1d0..fb501a1bc2 100644
--- a/actioncable/test/channel/broadcasting_test.rb
+++ b/actioncable/test/channel/broadcasting_test.rb
@@ -26,14 +26,23 @@ class ActionCable::Channel::BroadcastingTest < ActionCable::TestCase
   end
 
   test "broadcasting_for with an object" do
-    assert_equal "Room#1-Campfire", ChatChannel.broadcasting_for(Room.new(1))
+    assert_equal(
+      "action_cable:channel:broadcasting_test:chat:Room#1-Campfire",
+      ChatChannel.broadcasting_for(Room.new(1))
+    )
   end
 
   test "broadcasting_for with an array" do
-    assert_equal "Room#1-Campfire:Room#2-Campfire", ChatChannel.broadcasting_for([ Room.new(1), Room.new(2) ])
+    assert_equal(
+      "action_cable:channel:broadcasting_test:chat:Room#1-Campfire:Room#2-Campfire",
+      ChatChannel.broadcasting_for([ Room.new(1), Room.new(2) ])
+    )
   end
 
   test "broadcasting_for with a string" do
-    assert_equal "hello", ChatChannel.broadcasting_for("hello")
+    assert_equal(
+      "action_cable:channel:broadcasting_test:chat:hello",
+      ChatChannel.broadcasting_for("hello")
+    )
   end
 end